How each goes about taking their first d☆ck. Unless otherwise noted, they're on top and in control of the pace, and have either never been penetrated before or at least have never been penetrated by something bigger than two of their fingers. And of course are with their bro.
I'm boring and think of vajimbos but feel free to imagine a butt instead.
UT Sans: Agonizingly slow. Needs to rest after nearly every centimeter. Tries really hard not to breathe weird or make noise, unwilling to admit he's not just messing around teasing. By the time he's fully seated, he's out of breath from holding it for so long. Won't admit how overwhelming it is, but can't help but cling to Pap's ribs.
UT Papyrus: Worked himself up thinking it was going to hurt terribly, and the discomfort doesn't live up to the hype, so he takes it smooth and easy. Doesn't stop; finds the continued motion helps dull the slight pain.
UF Sans: It's a heat-of-the-moment thing, so neither of them are really thinking about it when Papyrus pushes him down, and he's surprised by just how much it hurts and ends up kicking Papyrus in the face. Thought lube was for quitters, so they don't have any, and he grumps around while waiting for Papyrus to get back from the store. Papyrus yells about the huge mess Sans makes using more than half the bottle all at once, but Sans snaps back, "shut the f☆ck up. what you gonna do, f☆ck me to death and use me like sawdust to clean it up?? friggin chad thunderc☆ck's out for blood over here, if he can't rip through your taint, he'll bludgeon you with his damn python." and squeezes out the rest of bottle just to be a shit. It still hurts like a motherf☆cker, of course, and Papyrus lets Sans beat on his chest to help distract from it. Says there's not going to be a next time, but changes his mind about half-way through the night.
UF Papyrus: Learned from the above disaster. Brags about how he, unlike Sans, is not going to waste half their night on something so trivial. Is pretty good about hiding his discomfort, and even tells Sans he was being dramatic that time. The banter is a good distraction from the fact that he's not quite f☆cking Sans stupid like he said he would. He'll make good on that promise next time.
US Sans: Doesn't expect it to hurt, so he goes at it with a little more gusto than he should. Very vocal. He pauses after the head, but he doesn't stop for long, not wanting to back down. Nothing he can't handle!!! Has to be told to slow down.
US Papyrus: Tries to play it cool, letting Sans take the lead, but he's actually really worried about it hurting. Tries to endure, but he curls away with a pained groan and worries Sans, who pulls back out. Papyrus ends up crying just a little from a mixture of pain, embarrassment and feeling like he's ruined the evening. He admits he had been looking forward to this, even though he was nervous. Sans calms him down, offers to stop or keep going. Gently works him open until they're snugly joined.
(But I also just like to think of this Papyrus as delicate. (*pq′ー｀) He never fully adjusts to how girthy Sans is, so it always hurts at the start even with extensive prep. He learns to not mind it, and then enjoy it.)
SFR Sans: Sets the evening up as a very slow, sensual, teasing atmosphere. Tells Papyrus he isn't allowed to move. Distracts them both with kisses so neither of them are thinking too hard about how slowly he's working himself down. It actually hurts like a b☆tch, but he's glad to feel it. Glad they can share this.
SFR Papyrus: Takes it all in one go, slamming down, groans and comes immediately. It feels like something tore. He's crying. Sans, worried, asks what he should do, but he takes so long to answer. When he does, he just begs Sans to move. It hurts too much to move on his own, but he doesn't want to stop. He doesn't stop crying. It'd be less awkward if he could articulate that he likes how it hurts, but he's just too overwhelmed at the moment. He's sad that it'll never hurt that same way again.
SFP Sans: Most of the time is spent just looking at it. Snaps "DON'T RUSH ME!" when questioned. It's annoyance that gets him to actually do anything, but he needs to pause after a couple inches. He stalls once it's all in, not wanting to admit he needs time to adjust.
SFP Papyrus: Lets Sans do what he wants. Tries to cover up his wincing with complaints about Sans's lack of a delicate touch, but Sans goads an embarrassed admission out of him that it's his first time doing this.
FSG Sans: Sans had asked Papyrus to lead and planned to endure any pain for his brother's sake, just wanting him to feel good, but Papyrus refused, too worried that he'd hurt Sans even with instruction. Sans still largely sets aside his own discomfort, taking it a bit faster than is comfortable for the sake of assuring his brother he's fine.
FSG Papyrus: It's an all night process. He flinches away once it starts to hurt and needs a break. Sans keeps asking if he's sure he wants to do this, but he's determined. They eventually get him fully seated, but it's still too much to really work with, so they just sit still for the rest of the night. He's too sore to try again for a few days, and the next attempt goes about the same. Fourth time's the charm, and he apologizes for the trouble, but Sans assures him he's enjoyed simply being joined with him plenty enough.
HT Sans: Lets Papyrus do what he wants. A sharp inhale makes Papyrus stop, ask if he's alright, but Sans shortly just tells him to keep moving, that it's going to hurt no matter what anyway. Just do it. Papyrus tries to go slow anyway.
HT Papyrus: Is so enamored with the idea (and used to most things hurting) that he doesn't really notice it or let it slow him down.
UL Sans: He was begging Papyrus to take him anyway. He's not going to make him stop now. He cringes and groans loudly, and Papyrus tries to pull out, but Sans locks his legs around him and says it's fine; it's supposed to hurt. A little teary when he asks Papyrus to please keep moving.
UL Papyrus: Kind of regrets just going for it, but stopping part-way would hurt worse. 'Fights back' against the pain by bouncing harder. He'll have given himself a limp by tomorrow.
G Sans: Lays back and is super flippant about it. Pokes fun at Pap for putting so much effort into preparing him. Stops laughing when Pap nudges inside, whimpering a bit. When asked if he's okay, he tries really hard to play it off, says he was just trying to get a rise out of Pap since he cared so much. Papyrus sees through his bullsh☆t, kisses him sweetly, and kindly says he'll be more gentle. Sans grumbles to himself that Pap is way too suave, but he's putty in his hands already, wrapping his arms around his brother's neck and melting into how much tender care he's receiving.
G Papyrus: The only one of them to have used a reasonably sized toy first. Slowly acquainted himself with the sensation on his own so he wouldn't end up being the reason a nice evening needed to stop or pause. Sans is pretty salty about it since Pap made Sans's first into such a damn event.
US Sans (alternate): It doesn't hurt at all. Papy tragically has a pencil d☆ck. Since he takes it all at once, Papy expresses concern for him, and Sans has to make the snap decision on whether to be honest that he can hardly feel it or pretend in order to save Pap's ego. (If the truth ever comes out, Pap will be devastated, ask why Sans let it go on that long, and Sans responds "BUT YOU WERE SO CUTE, THRUSTING AWAY LIKE THAT. LIKE YOU WERE GETTING SOMEWHERE. IT WAS REALLY CUTE, PAPY." and like, it's not like he wasn't laughing at him, but it wasn't in a mean way, but it's difficult to get that across. For Sans, there's something endlessly endearing about Papyrus trying so hard but being unable to pleasure him. He's not sure what this k☆nk is.)
UF Sans (alternate): Wants it to hurt. He spends his life trying so hard to avoid getting hurt, and he knows Papyrus would never harm him. He wants to feel his brother safely break him. A wonderful pain that symbolizes their trust and love. He urges Papyrus onward, freely cringes and groans in pain, pulls Papyrus closer when he tries to pull away, begs him not to leave, to keep going.
