In Snow Crash what is the significance of Y.T. having sex with Raven?
This is probably a reflection of their moral ambiguity, and possibly as a commentary on human sexuality. There's a minor chance that it's deception as well.
Y.T. had previously considered her attraction to Hiro, but discounted that anything would happen between them because he doesn't strike her as the sort of person who would have sex with a 15 year old. It isn't clear from the context whether Y.T. thinks this is a matter of moral judgment, or of Hiro simply not being interested. She also doesn't seem to be very compatible with boys her own age, as evidenced by her awkward interaction with her boyfriend.
Raven was attractive to her the moment she saw him, and she doesn't seem to consider their age difference or the possible danger he poses (in fact the danger is part of her attraction to him). So it's pretty clear that she is genuinely interested in him sexually, and that this is why she's hanging out with him. Amid the general squalor of the rest of the Yacht compared to the Core, and in society in general, going on a "date" with Raven may be one of the only times that Y.T. has been able to experience a better quality of life. Ironically, then, sex (which the author describes as being incredibly fulfilling for Y.T.) which is as un-civilized an act as one can imagine, may be a commentary that humans can still find pleasure in simply being human, despite the society they create.
Finally, in a sort of post-coital haze, Y.T. remembers seeing Metaverse terminals and wishing to get information about Rife's location to Hiro, but knowing she would need to incapacitate Raven in some way in order to do so. This leads her to realize that she may have subconsciously "forgotten" to remove her dentata, which does result in Raven being knocked out. However I think it's a stretch to say that this omission, as well as the sex, are part of some sort of elaborate espionage that Y.T. is doing subconsciously.