Also, as wedrifid said, some people are born with their sex unclear. Often, surgery is performed to "correct" the baby to a particular sex.

I think that less constrictive gender roles would solve most of the social pressure in those circumstances.

How so? I don't think a weakening of gender roles will help to change the fact that most people aren't going to be sexually attracted to someone who's biologically sort of in between the sexes. Or are you referring to a different social pressure than the one towards surgery?