Di Vinci explains it all

It took a quite a long time before Ritsuka finally began to flag. Not in stamina or enthusiasm, he was more than willing to go on and keep fucking her for another hour or so after this. But he was looking at Mordred, and the simple fact of the matter that she looked like she could go on much further, and if he wanted to submit to her, he was going to have to leave now and let her take full control over.

He began to draw out his cock from her throat, but it was a slow and agonisingly pleasurable process, and she fought him every single step of the way. Her mouth was just clamping tightly down on him as he pulled more and more, drawing his length out of her throat slowly and steadily. Until finally it pulled free, followed by a large dollop of cum that was squeezed out after him.

"Mordred." He called out meekly, and the Blonde servant looked up at him through half lidded eyes, her face on the edge of falling into the state of a lust rampage that Bradamante had been victim to for the last hour. "She is all yours. Knock yourself out." That news brought a flash of light back into Mordred's eyes, and she rapidly yanked her cock free of the Lancer's cunt, allowing her to fall down onto the ground and collapse onto the floor. Mordred basically teleported around to the frontside of the servant and grabbed her mouth before she could drop fully, and he watched as she began to line herself up.

He turned away and began to leave, which meant that he didn't see when his Saber plunged into his Lancer, but he did hear the sound of more dollops worth of semen glopping out of her used mouth, displaced by the mighty girth of the conquering cock. What his eyes were paying attention to was the complete lack of Mashu over in the corner. She must have left some time in the last half an hour while he and Mordred were fucking away. That could be bad. Or it could be good. He had know, it wasn't exactly the simplest of situations.

What he did know, was that he didn't know the first thing about where to go if he wanted to find Mashu right now. She knew Chaldea a thousand times better than he did, and if she didn't want to be found then there was no way he would find her unless Fou decided to do him a solid. He looked both ways down the corridor. No sign of the little squirrel-dog thing. Fine then. That meant that he could get to know a few things that he had wanted to for a while now. There was no point heading to the command room when there Bradamante was so insensate, and with Mordred following him all the way in astral form, anything to do with the Singularities basically could be left for Later

So

With that in mind he headed over to the one place he could get the answers he was seeking right now."

Hi Hi! Welcome to the Glorious Da Vinci's Workshop! Take your time browsing as you please!" The genius inventor called out even as she span around this way and that, turning between 5 different trinkets that were whirring, sparking or otherwise doing invention sort of things, the genderbent hero switching between all of them without pause and tending to them all as they needed. He took the time to do as she instructed and looked around the workshop. There were shelves covered with contraptions, desks and other worktops piled with inventions, and even a reproduction of an early plane prototype hanging from wire from the roof.

"Actually, I came to talk to you." He said even as he moved past a desk, dragging one finger along the worktop past a small stack of golden cubes, a much large pile of green ones, and a set of 5 golden ticket like things that were splayed out like a hand of cards. He moved past them all and headed to a small seat that was set against the wall at the side of the room, and sat down in it, glad to be able to take some weight off of his feet after so long.

"Oh?" Da Vinci mused, twirling on the spot again and setting all of her trinkets to a stable state, before turning and coming over to a desk right in front of him, bending over at the waist to lean against the top. Her elbows came down onto the worktop, and her hands clasped together in a little boat, which she rested her chin into like a little seat for her head. It also provided the perfect view between her steepled arms down into her cleavage. A genius like her probably knew that though. 'Wife, I am going to make this servant my Broodmare' Mordred stated through the master/servant link. He had to **** himself to not get hard.

"The briefing you and Dr Roman gave me never said anything about servants being Futas. And twice is a little more than a coincidence." He said simply.

"Oh. That. Why that is quite a simple matter for a genius of my calibre! It's a purposeful glitch!" She said happily like it was a little interesting nugget to brighten up her day. "When we repaired Chaldea's systems from the bombing we used the grail that we recovered from the Fuyuki Singularity. Using its power, the hardware of a lot of Chaldea's systems was repaired instantly, and so we mainly just needed to recalibrate the software to get everything working again. I was occupied at the time fixing the Rayshifting capabilities and making sure that all of the Masters injured in the bombing were safely put away into Cryosleep. So repairs to systems like the Servant Summoning system was left to the normal staff."

"And they made a mistake. They can't truly be blamed. No one left on the staff is working in the positions they were hired to fulfil. Everyone is stepping up to take the place of all of those killed in the bombing. But we make do with what we can. They wired the recovered Grail directly into the Summoning System due to the Grail's natural servant summoning capabilities, to sort of hijack its power to jumpstart the summoning system from scratch. But the Grail was flawed. Or maybe rigged you could say. It can only summon Futa servants for some reason. I suspect that if the other Singularities also contain grails, then all the Servants within those will be Futanari as well. But honestly there really is no need to fix such an simple problem. The summoning system is functional, and to undo it would require dispelling all servants summoned since the bombing, disconnecting the Grail and initiating a full purge of the system code to make sure nothing is left behind. And all of that just so Servants are summoned as the correct genders? It would really be going to huge expense just to fix a tiny issue. If a Hero can't deal with their genitalia being reshuffled." She said, making a point of looking down at her own buxom body, reshaped to look like the Mona Lisa that Da Vinci had always hailed as the true epitome of feminine beauty. "Well are they really even a Hero?"

"I suppose I see your point. Mordred doesn't have any problems with being a Futa now at all. And Bradamante is mostly herself it seems, just with extra. I do have another question though."

"Oh do you? Well then I suppose I shall answer. No, neither Mashu or myself are Futanari. We are in fact the only female servants that Chaldea shall possess until the Incineration of Human History is finally resolved. Isn't it extraordinary?" She eyed him teasingly, and shifted a little so that he cleavage bounced within the gap he could see it through, breasts rubbing together as if mimicking the motions they would make when squeezing around a cock. "If Chicks with Dicks aren't your thing, then you will have to turn to either your Genius Big Sis Da Vinci-chan, or to your ever dependable Kouhai! So, which of us shall it be, who shall you slake your lusts upon?" 'I know who I am going to slake my lusts upon.' commented Mordred.

"No, that wasn't-"

"Fooey." She said, batting a hand at him. "Let me tease you a little first! I know you like your women like that, with the extra equipment. You wouldn't have dived in so eagerly with Lancer if you didn't." She said, letting on that she had apparently being watching in on the events of the Summoning Room. "Watching you three go at it was definitely appealing to dear old me, but not whatever shall I do? Without the proper equipment, our only Master will never look at moi with the same eyes." She brought one hand to her brow as it to fake a feint.

"No. That isn't it at all. I meant to ask what was up with Mordred suddenly becoming so dominant to me once she fucked me enough. She is a servant and she did take control as she wanted to."

"Oh?" She said, actually looking surprised and perplexed for a moment, before finally answering him. "I assumed that you had encountered the Futa fetish before, given your submission to Mordred. Now you say you don't know? Well then let Big Sis Da Vinci enlighten you!" Quick as a flash she slipped a pair of glasses out of a pocket and mounted them on her nose, a pair of perfectly fitted spectacles that made her look far more respectable than she had a moment before. "Futanari, are inherently different from normal humans in a few ways. Most of it comes down to the details of their biology to make such an existence function, but the difference you have stumbled upon, is instead a more instinctual, or behavioural difference."

"Futa, are inherently a Hierarchical gender. When left alone with each other in large groups, a feudal-esque structure will always emerge within the group, with some on the bottom, and some on the top. These societies, depending on the personalities of the members in the upper echelons, can function either as a distinct ladder, where every member knows exactly where they stand in their hierarchy, every one of them knowing which rung they are on, who is above them, and who is below them."

"The second possibility, is a more feudal triangle, where the 'ranks' so to speak, are less strictly defined. There will be one member who clearly sits at the very top of the pyramid, unchallenged by all, and every other member knows to respect. But below them will be a whole slew of Futa who are all approximately equal in rank and station. Their may be favorites, or some who know their exact position with respect to a specific other futa, but on the whole they are just understood to be the upper class."

"Below them, you will find the same situation repeated, with a mass of futa who all know they are the middle class, but none being aware of how exactly they stand within the hierarchy, beyond that they are middle class, and thus above the lowest class, and below the upper class with one exception with Mordred above you."

"This all calls back to how these ranks and classes are decided. And that is through contests between individual futa. One on one matches of physical prowess, whether that be in battle, or in the bedroom. Either can function for the same result, and in fact occasionally jockeying can occur for the same position when you have two futa, and one has the advantage in battle, while the other has the advantage in sex. There is no particular emphasis on Sex or Battle over the other, except internal cultural pressure from that particular group, there is no universal standard by which the futa decide."

"This of course explains the two different structures their societies can take. In the Ladder system, every futa has competed with every other, and therefore knows exactly where they stand. In the pyramidal systems, there is uncertainty, because the competitions have simply not been as prolific. And then we have the situation with you."

And with a flick of her wrist, images began to be projected onto the wall beside her, and she half turned to look at them. He leaned forward to get a better look too, and saw that they were all still images of him getting fucked by Mordred and fucking Bradamante. "You as the Master, are naturally expected to be dominant, to be in charge, to be the one to give the orders, despite the fact that you are inferior in pretty much every way to the beings you are contracted with."

"And Mordred knows this. As a Servant it is a fundamental part of their existence in this world that she be subordinate to you. But it is also a fundamental part of her nature as a Futanari that she must discover her place within the hierarchy. Because of these two facts, she appears to have reconciled the two conflicting natures by challenging you in much the same way she would another Futanari. This resolution was likely driven by several factors, at least one of which may have been her own personality and nature as the Knight of Betrayal, driving them to rebel against your Authority. But whatever the reason, Mordred drove to dominate you and assert herself as the dominant one, not just in your relationship, but as the head position of the hierarchy."

"And then you lost. You lost to Mordred in sex, and that, combined with that she is already supposed to be superior to you, has inserted herself into the hierarchy as the highest member in her eyes, and already you have enforced that ideal onto her through your combined domination of Bradamante. You shared her yes, but she took the part of her she desired, and offered you the parts she didn't. You fundamentally asserted dominance over one of them and submission to the other at once, by enforcing Bradamante as the bottom rung of the ladder beneath you and Mordred above you."

She turned to look at him, one hand cupping her own cheek as she looked him in the eyes. "My my. I must say, if you have really been doing all of this without any knowledge, it is almost frightening how well you may perform in the future now you do. So long as you continue to solidify your position as the second of the hierarchy with all future servants you summon, you would not only secure Chaldea's internal stability despite all of the disparate personalities of all of the servants we are to summon, you may very well turn Chaldea itself into Mordred's personal Futa Brothel. Keep this up, and every servant will be just a part of Mordred's harem. I'm almost jealous."

"I-is that... Is that all true? It sounds almost like the plot of some hentai doujin."

"Oh yes, I was being quite candid with you. And being candid again, it is honestly quite a good thing that you seem to have a large thing for Futa, because it is highly likely that every servant you encounter from this moment on until the Incineration Event is resolved will be a Futanari. Anything related to the Fuyuki Grail, and the other grails that are causing the other singularities will be tainted by their hermaphroditic influence. Even for a genius like me, that could be overwhelming." The Caster servant straightened out her back, and slowly walked around the desk to reach him.

He looked up into her blue eyes, her perfect face, all framed by her beautiful chocolate hair. She was as beautiful as the Mona Lisa painting her body had been created to mimic. And she smiled down solemnly at him like she was a nurse trying to comfort someone who they knew was suffering from something terminal and they hadn't yet told the news to. Slowly she crouched down, and wrapped her arms around him, pulling him forwards until his head was nestled into her chest, and hugged him there, stroking one arm down the back of his spine.

Ritsuka sank into the embrace, Da Vinci just as genius a hugger as she was at everything else. She had it down to an art. "I'm sorry. We're all putting everything on you now. Everyone is relying on you and you alone not to fail. No matter what goes happens, I promise you I will do my best. And if you ever need anything, my Workshop is always open to you." Slowly she pulled back, planting her hands on his shoulders, and looking him dead in the eyes. "You're not alone. Everyone is placing their faith in you, but they don't need you to be perfect. You may have to suffer in silence, and grit your teeth and bare with things you shouldn't. If you need help, it will always be given. You only have to ask."

"Take this as a start." She said, and he felt as something was pressed into his hands. Looking down, he found she had handed him a bottle of water, though the label said it was special Da Vinci style water, filled with all the minerals and electrolytes needed to hydrolyse, energise and keep someone healthy throughout sex marathons. "Just please try to hold back on the orgies until we've recovered another grail. Right now the power supply isn't enough to be able to just pump this stuff out. And You may want to drink a little now." Da vinci stated with a grin.

"Thanks Da Vinci." He nodded, hand tightening around the bottle lid. He was probably going to need it in the future. Slowly he slunk out of his chair, and made for the door, moving to leave and get on with something, anything really. But at the door he lingered for a moment, and then turned over his shoulder to look at Da Vinci who had already turned back to her experiments. "Wait. Were you being honest when you said you'd let me fuck you?"

No answer came to the question. After almost a minute of standing awkwardly in the doorway, Ritsuka left and took a sip but didn't realize that Mordred materialized behind Da Vinci with her cock fully erect with Da Vinci fully unaware. Little did he know that something inside his body began to change.