Sure

Maybe if you allow Alyssa to get the sex with Sid out of her system, then she will eventually want to come back to you. So you say yes, not wanting to interrupt the process. Surely if you say no, she'll just want him even more.

Alyssa: "Thanks babe, it's going to be soooo much fun..." She says, holding her pussy shut with her hand to keep Sid's cum from leaking out.

You: "Just until you're done with the lessons, right?"

Alyssa: "hm? Oh! Yeah...the lessons. Sure, baby."

Sid: "Thanks a lot for being so understanding, Matt. I'm gonna have such a good time fucking your girlfriend in your bed, every day."

You: "yeah....sure"

The rest of the day actually went by pretty normally. Sid and Alyssa, after cleaning themselves up, helped you organize the new stuff and move your stuff from the master bedroom to the guest bedroom. The whole time you felt like Sid was actually replacing you, first in Alyssa's sex life, then your literal place in your bed at her side.

You then got takeout and watched a movie. Alyssa cuddled Sid the entire time, her rubbing his chest and hairy stomach, while he massaged her big breasts, and they kept making out during the movie. You sat on the lounger by yourself, your dick getting quite hard at all the heavy petting going on in the sofa.

Before the movie ended, Sid and Alyssa were fully making out. Sid had ripped Alyssa's top off, revealing her aroused big breasts. Alyssa responded by tugging his sweatpants down. Soon the were fully nude, sucking and licking each other's genitals on the sofa.

It's not long until Alyssa straddles Sid again, lines his rock hard cock with her pussy, and let's herself fall down, penetrating herself with Sid's big cock.

Sid: "mmmm yeahh...that's my girl..."

Alyssa: "I'm your girl...baby..."

You clear your throat, as if to remind them of your presence. Alyssa is still your girlfriend, after all.

Alyssa: "Sorry, Matt. Just...gnn......"

Sid, apparently trying to cover up for Alyssa's loving comments towards Sid, says: "Yeah...I instructed Alyssa to...pretend I was her boyfriend....yeah....that way she can practice mentally to better please you. If she thinks she's fucking another man, she would think she were a slut...and we all now she isn't." Sid's sentence was punctuated by his cock going balls deep inside Alyssa, literally hammering the irony of that statement in your girlfriend's unprotected pussy.

Alyssa: "Ughnnn...yeah..that's what I meant..."

You: "oh..."

So, Sid and Alyssa have unprotected sex in front of you, once again. Sid told you to get the fake pussy you bought, and follow along. For learning....you were actually glad, and ran for it. Lubing the fake pussy up, putting the condom on, and placing it on your dick, the same way Sid's cock was up your girlfriend's real pussy.

You watch them fuck, and move the fake pussy in rhythm on your dick. After a while of the familiar sounds of moaning, grunting and flesh slapping against wet flesh, Alyssa's toes curl in a delicious orgasm. Her convulsing pussy must have pushed Sid over the edge, as he calls out: "Uuggghh...I'm cumming....I'm cumming...!"

Alyssa: "uunnnnfff ...cum in your girlfriend's pussy, baby, cum for meeeee...."

Sid's balls contract up and you can see his cock spasming in Alyssa's pussy again and again. Delivering his hot cum into her unprotected, fertile womb once again.

You stop fucking the plastic pussy for a moment, they started ahead of you, so you're a little ways off from cumming. "Uhh...this is still...pretend, right?"

Alyssa: "Yeah, Matt. Just pretend.... nothing to worry about...." She says as she french kisses Sid while his cum leaks in between the walls of her pussy and Sid's cock deep in her pussy still.

Sid: "Yeah.... we're just pretending....so from now on, I'm Alyssa's boyfriend, and you're....just Matt, our roommate....now...don't you want to cum, Matt? Show us how you fuck that thing ..."

You look away, and start fucking the fake pussy again, while Sid and Alyssa keep making out, enjoying the afterglow of their lovemaking session. After a few minutes, you pathetically cum inside the condom, and set the fake pussy down. Alyssa and Sid get up, and run to the shower together, laughing and slapping each other playfully. Sid's cum leaks all over the place, out from Alyssa's pussy.

Alyssa: "Can you clean that up, Matt? Thanks!"

The lovebirds take a long time in the shower, so you have plenty of time to clean Sid's cum off of the floor.

And so, that becomes the new normal around the apartment. Sid is Alyssa's pretend boyfriend, although Alyssa seems to take the role quite literally, leaving you completely without any sort of affection, giving her all to Sid. But Sid convinces you it's all part of the act, and not to worry. You vent your frustrations out on the fake pussy, most often in the private of your room. The plan was to learn and imitate Sid and Alyssa's positions during sex, but Alyssa said it weirded her out seeing you fuck that thing. So you had to just watch them fuck and then try to do the same thing by yourself.

Every day, Sid and Alyssa have unprotected sex, all over the apartment with absolutely no shame. Sometimes on the kitchen table while you're eating breakfast, in the washroom on your clean clothes, outside your bedroom, everywhere. Sometimes they want privacy, so they lock you out of the apartment for a few hours while they make love alone. "Sorry, bud, Alyssa sometimes wants some privacy while she has sex with her boyfriend....me." says Sid as he locks the door on your own apartment. You had to sleep on a bench outside that night, since they forgot to let you back in.

All the things you used to enjoy with Alyssa, have stopped now. One time you asked Alyssa if she wanted to shower with you. She paused, and Sid answered: "You want to shower with my girlfriend? I know she's beautiful, but that's not okay with me. Call me old fashioned, but no."

Alyssa: "Sorry, Matt. My boyfriend said no."

You chalk it up to the pretend act that's been going on, soon it will end, surely. Meanwhile, Sid and Alyssa keep having unprotected sex every day.

One day, with all of you on the sofa watching some TV, Sid and Alyssa seem to be gearing themselves up for another lovemaking session.

You: "Uhm, Sid? When are Alyssa's safe days up? You've been...risking it for a while now..."

Sid: "Well...not that it's any of your business, but Alyssa has agreed to have my baby. So I'm actually trying to impregnate her." He kisses Alyssa as they spring the 'good' news on you.

You: "Wait, what??"

Alyssa: "I guess we better come clean..."

Sid: "Yeah....so, when we said Alyssa was my girlfriend, not yours....we weren't pretending. You guys are broken up, have been for a while now." Sid says as he massages Alyssa's naked breast.

You: "I....can't believe it..."

Sid: "You let it happen, bud. I guess in a way, you knew I deserved Alyssa more than you. We're all happier this way. You can be my best man at our wedding, how about that?"

You: "You're getting married??"

Alyssa: "Can you believe it? He proposed to me the other day. Of course he can't afford s ring, but we'll manage." She kisses Sid affectionately.

Sid: "I was actually hoping you'd buy the rings for us. As payment for all the lessons I've given you guys over the past few weeks."

Flabbergasted, you.