She wants to play!

You look at Kako a bit. You figure you can play along.
"See, I am not embarrassed!" You turn and hike up your skirt in the back until it is around your waist. Then you say, "See my panties? There they go!" You pull down the waistband in the back and tuck it under the curve of your ass. You do this slowly and with plenty of rubbing of your naked flesh. You shake your ass lewdly for extra effect.
"Sir!" Kako gasps. "I am appalled! You show your underwear and then your naked flesh! That is so rude...even if said flesh is very attractive."
"Look, I'm just trying to help. I was just showing you that revealing yourself to me is nothing to be ashamed of since you need to change your soaked panties. You don't want it to soak through to your dress, do you?" Kako seems to think for a moment.
"You are right, of course. Soaked panties are my problem, but wet spots on my skirt are not proper." She hikes up her skirt in the front to find that the wet spot has grown sizably. She hauls out her penis to find precum flowing freely from the slit. "My dick won't stop leaking," she says with a frown.
"That's not proper for a lady." You gyrate your naked ass a bit as you feel it up. You like feeling up your own butt. It sticks out so far and it is so wide and plump. "Now why would that be? Does this happen often?" Kako shakes her head.
"No, it usually stops soon after completion. I think your lewd ass gyrations have aroused me. While it is very improper to expose yourself in that fashion, I must say that you are quite the specimen. The only way to get this thing to stop leaking is for me to achieve release. Since you and that ass have done this, and you want to help me so bad, you can have anal sex with me."
"You want to fuck my ass? And cum in it?" Kako shakes her head again. But this time it is her cockhead, and each time she does it slaps your buttocks and leaves a spot of clear precum that is so large it runs downwards. Many of these soon cover your large protruding bubblebutt. Each slap causes your ass to ripple in a pleasurable way.
"Yes, I intend to penetrate your asshole and inseminate it. I would normally only need one climax to stop my penis from leaking, however, you have aroused me to the point that I may need to orgasm many times to get my lubricant to stop flowing. I may end up ejaculating a very large quantity of semen into you. Are you sure you can handle that?"
"Yes, I can handle all of the cum you may need to let loose." You try to spread your ass cheeks for Kako only to find them so slick with her precum that you cannot get a grip! You just smear the loads around as much as possible to make your ass shine. Being fully slippery on both ends, Kako's cockhead parts your buttocks without much effort and she puts her hands on your hips to pull you back as she pushes forward. Your asshole parts for her when you let it relax. You had her poking at your hole a bit to start. You wanted to make sure that she was good and ready when she finally entered you so that she would start cumming quickly.
"Here comes the first batch. Enjoy!" Kako holds her position with just her cockhead inside of you. Her fountain of cum fills you quickly and starts your belly expanding outwards. The shirt which seemed to fit so well tightens, but then stretches. It even feels like it is supporting your growing frontal protrusion, preventing the normal sagging. After her first load comes to a stop, Kako starts fucking your ass. Her crotch colliding with your wet ass makes squishy slapping sounds. Every so often a bit more length finds its way into your ass. After five minutes or so, she holds still against your ass and orgasms again. Gouts of her cum flood into you causing the familiar growth of your belly you have come to love. Looking down between your parted breasts, you can see your shirt still supports you well. You feel good about how much better it makes your bulge look. You are shaken out of your thoughts, quite literally, by Kako's powerful thrusts as she resumes fucking your ass. She really lays into your ass this time. Her panty-enclosed balls collide with the backs of your thighs. You watch your bra-enclosed boobs jump with each impact. This is no small feat considering how heavy they are! Your revealed cleavage jiggles. You really do have a nice rack. Your belly shakes a bit, but the tight red shirt containing it keeps it from going out of control. Kako almost pushes you off of your feet with one particularly severe thrust at your butt. You widen your stance only to find that her balls now collide with your own when she comes forward. That is just the icing on the cake to feel your swollen balls jiggle and swing with the impact. It seems like half an hour has passed before you feel the first twitches of Kako's cock that mean impending cum. You miss the collision of her balls with yours as she pumps her third load into you. Your belly grows again, but the shirt keeps it from sagging too far. Most of the mass expands upwards between your spread tits and occupies the area that they normally would if they weren't pulled apart by your new bra. Now you see why it is made like that. It makes room for your cum-filled middle to grow into.
"So, do you feel your cock still leaking, Kako?"
"Yes, it still is. I told you that it could take many orgasms. Just stand there and continue to be helpful. I will let you know when I feel that I can safely withdraw."