The light is no mystery.

YESSSS thank you anon! I have two different headcanons for them, one if they’re together pre & during the war, one for if they don’t get together until the modern era. I am going to answer based on the first one!

What was their first kiss like: It was really late at night in their tiny excuse for an apartment. Bucky had been out late and gotten into a fight at a bar, and subsequently was sort of sniping at Steve because he was dealing badly with all his repressed feelings for his best friend. Meanwhile Steve is all worried about him (because Steve was and will always be a mother hen) and is convinced that even though Bucky likes guys Bucky would never want him. So when Bucky finally snaps at Steve about having feelings for him, Steve is like, “Wait what? But you can’t, why would you want me?” and there’s a confused moment where Bucky assumes that’s a rejection until they get it cleared up that Steve just has serious image issues. And then Bucky kisses him, really gently the first time. Steve sort of clings. They heart each other, those boys.
Where were they their first time having sex: Their apartment.
Who’s louder: They have to keep quiet, because hello, it’s the 40s and also because the walls of their place are paper-thin. But if that weren’t an issue it’d be Bucky I think. He strikes me as vocal; Steve not as much.
Who wakes up first: Bucky. Pre-serum Steve is always tired from the 54673840000 things that are wrong with him, so he tends to sleep a lot and have a hard time getting up in the mornings. (Post-serum he’s up at the crack of dawn though).
Favorite form of foreplay: Fffff well. I think Bucky can be a severe tease sometimes, getting Steve all turned-on and playing dumb about it until Steve wants him badly enough to get over his shyness about sex and ask. But for Steve it’s something simpler, kissing or just being physically close.
Who performs/receives oral more often: I think Bucky does it to Steve a lot because he loves how it always drives him crazy, like every time is as amazing as his first. That said, Steve likes doing it because he really likes making Bucky feel good. He’s not entirely convinced he’s good at it, I don’t think, but Bucky always has the opposite of complaints, so. TL;DR THEY BOTH DO.
What kink they most often use: They’re fairly vanilla on the whole. Bucky has a size kink, though, in either direction– small Steve and buff Steve both have their appeal.
Who more often tries new things: Well, it’s all new to Steve when they start, so by definition it’s him.
If they had to choose a third+ person to include who would it be: Peggy.
Favorite toy: N/A, they don’t use them
Is there a certain thing one of them can’t have sex without: the other not really.
Their general feelings on sex in the relationship: Um… yes please? Hah. Seriously, though, those two love each other like crazy, and the sex just becomes a part of that. They both enjoy it and have it whenever they can, but the important thing is that they’re with each other.
How careful are they about using protection: Not very. It’s not readily available to them given their circumstances.
A fantasy of one that the other will never be able/willing to fulfill: They wouldn’t want to do anything that would or might hurt the other one.

What was their first kiss like: Uh, awkward? Haha. They were quite young and didn’t really know what they were doing, so there was a lot of bumping noses and chins and inopportune laughter.

Where were they their first time having sex: Tony’s bedroom.
Who’s louder: It’s pretty much a toss-up, I think. Neither of them is quiet, except sometimes Tasha shuts up because she temporarily forgets how to breathe.
Who wakes up first: When they sleep, they both sleep late. Tasha is a very light sleeper post-Afghanistan, though, so she would wake at every little thing then.
Favorite form of foreplay: Being awesome with machines together.
Who performs/receives oral more often: Tony performs it more. Tasha will do it, she has no qualms about it or anything, but it’s not her favorite thing. Tony loves doing it, though, which works out just fine for her!
What kink they most often use: They’re kind of one another’s testing ground for sexual experimentation, so they’re tried a LOT. Bondage is the one they always come back to, though; they’re both really into it.
Who more often tries new things: It’s pretty equal.
If they had to choose a third+ person to include who would it be: They’ve done this. Uh, kind of a lot. Notable ones: Rhodey when they’re younger, later Steve.
Favorite toy: Changes constantly. They’re inventors and they get bored.
Is there a certain thing one of them can’t have sex without: Uh, no? Well, post-Afghanistan Tasha strongly prefers to have some light.
Their general feelings on sex in the relationship: Hah, well. They obviously know it’s weird and taboo and generally Not Okay, but basically they both end up pretty much going, “fuck it, it’s consensual and it’s not hurting anybody,” and letting it go. Though of course they keep very, very quiet about it. It’s not like it’s a regular occurrence, anyway.
How careful are they about using protection: EXTREMELY.
A fantasy of one that the other will never be able/willing to fulfill: I can’t think of anything offhand.