If you want to optimize for no->yes, which I presume you do, I would say start with something much less intimate, like kissing. And even then, you'll probably want it to happen when there's already some vectors pointing in that direction.

If you ask the sex question out of the blue / too early, then that signals that you just generally have sex on your mind, which in many cases is seen as a bad thing. And depending on tone it can convey lots of other undesirable ideas. There are also (many) situations in which your conversational partner may simply not want to say yes to that question too early, and therefore you don't want to ask the question yet.

It's... kind of like inferential distances? If your state of mind is all sexy, but your partner's is not, then it can be weird to just jump suddenly to sex. The difference (I guess) is that unlike just explaining something, there are indeed serious drawbacks (slaps, etc) from assuming the inferential gap is small. In general, I think it's a valuable skill to be able to quickly gauge where someone is on the inference or arousal spectra and respond accordingly. Perhaps separate skills though.