I think that not only a degree of buildup but also a narrative describing the aftermath is critical to making an enjoyable story.
The reason is that if it's all about sex, it's essentially sex with cardboard cutouts. We need to know a little something about who these characters are if we're going to be able to suspend disbelief and see them as "real" in any way. Even with someone like Lois Lane, a character who we're probably all familiar with, we need to get some established story going on with her; otherwise, she's just Lois Lane because you said she is, and the best the reader can do is imagine Margot Kidder / Teri Hatcher / Amy Adams playing her part (myself, I'd go with choice B). But, establishing a story for her, motivations, history with whoever she's shagging... that makes her more of a real person, and the more real she is the more sexually exciting she can be.
One way to think of it is to imagine two different porn movies. One of them features a pretty blonde and a pretty brunette, both with attractive but unremarkable faces and figures, approaching each other nude in a white room and having lesbian sex. The other movie takes five minutes to establish a female executive named Sally Thompson, who's dressed in a black suit and wears designer glasses, interviewing candidates for a secretarial position; in comes a woman in a low-cut top and high leather boots, and Sally realizes that she's a stripper who gave her a lapdance at a club the previous week. Eventually, Sally's discomfort tips off the candidate and she remembers the event, AND that Sally had let slip that she had always wanted to do something crazy like have sex in her office at work...
It's pretty obvious which of the above movies would be more entertaining to watch. Even though we watch porn for the sex, we generally want a LITTLE more out of it than just that. And that's even more true when it comes to written erotica. We're on the internet, here; if we were just looking for something to jerk off to, it's easy to find pictures and movie clips which require a lot less effort on the part of the consumer. We're here not just to read about sex; we're here to read stories. I would much rather read a well-written CHYOA story which takes ten chapters to get to the sex part, than one which kicks off with sex immediately but it's between characters we never learn anything about beyond dick length and cup size.