I agree. Apart from being a waste of perfectly good cake (although maybe the guy is into eating it afterwards), it doesn't seem like a big deal. But it's really not a first contact kind of question. Because, really, even something completely vanilla and not weird at all (such as "do you like being on top?") is not a first contact kind of question.
Maybe this guy has decided he absolutely cannot be with someone who isn't willing to step on cake, and he doesn't want to spend time investing in someone who might not. But it seems hard to imagine that someone would refuse such a harmless request.... once a relationship already exists.