I know what you mean. It's a visceral reaction.

I never point to gender dynamics and power issues, but I think they're to blame here.

With women, you don't usually have to worry about gender dynamics and power imbalances. Women are usually on even ground. Even when things aren't completely even, there's no unscalable gender wall. There's an overlapping understanding of what it means to be female, lesbian, whatever. It's a nice, protected safe zone of "I don't need to be guarded and explain anything to you. You already know." So, if your lesbian friend wants to comment on Betty's tits, well, it doesn't feel demeaning to Betty (aggressive, sexist, lesbians aside). You nod, agree: Betty does have fantastic tits.

With men, gender and power are always in the background. With heterosexual men, in particular, it's in the background and they don't even realize it, because (like most sane people) they think their way of thinking is the default setting on life. Worse: unlike you, most don't have any experience to suggest otherwise. Even when you know the individual very well--even when they are the most understanding and sensitive persons--trite gender expectations and biological differences insidiously creep in to the most inane subjects and act as a reminder: they don't really know a thing about what it's like to be you.

So, what happens when men try to reach you without destroying that gender-normative wall? When they approach you with all those insidious little gender preconceptions? It makes it feel almost like an invasion. Suddenly, your safe space has dirty boot tracks all over it. And again, they won't clean up after themselves, because they don't see the tracks at all. Suddenly, your safe women-only space is slathered with heteronormative male expectations and self-centeredness.

So your boyfriend comments over Betty's tits. Suddenly you explode about how Betty is more than just a pair of tits. You know, just in case he missed it.

But you know, you have to be fair to him, too. You have no idea what it's like to be a heterosexual man. Or any other type of man. And, it's not quite fair to expect him to react like a woman, either.