No more obviously unacceptable than bomb/murder jokes, both of which are pretty common.

Rape jokes are obviously unacceptable because they would violate even the flimsiest of sexual harassment policies.

Just because it might violate a sexual harassment policy doesn't mean it is "obviously unacceptable". Those policies tend to be very conservative.

In most workplaces (in my experience) people talk openly about sex and relationships, which would probably violate even these "flimsy" policies.

Sex is natural, and it's natural for people to want to talk about it. The important thing is whether you are deliberately harassing someone, whether you should reasonably know that your behaviour is abusive, and whether you stop doing it when you're told that it upsets someone. I've always worked in places where the majority of people are quite reasonable, but I realise not all workplaces are like that.