That may be all there's to it, but I bet the fact that it's sexual makes all the difference. Do you think Bucephalus355 would have bothered to complain about it if the scene was of a woman puking?

A) Puking is not anywhere near as disgusting, and you could get equally disgusted by seeing someone peeing on someone else even in a nonsexual context (e.g. hazing).

B) Some people can feel elevated disgust towards portrayals of other people being sexual. In that case, the fact that it's sexual does make the difference, but it's still a disgust reaction.

Point A is far more subjective than you're making it appear. Personally, puking is far more effective at provoking a reaction of disgust.