I don't think it's about expectations. In general, even as we've gotten more comfortable with sex in popular culture, our appetite for straight men discussing sex frankly is zero. Dan Savage said it best, "No one wants to read a straight man's sex column."
My point is there is a big "creep factor." If you are a straight man and you step out of the sexual box, people are going to cringe. That applies to straight men who like pregnant women as much (more so in some cases) as any other fetish. Just watch the "Two and a Half Men" clip where the guy is hitting on a pregnant woman. It's pure "creep factor." There's no playing-devil's-advocate or intellectual move you can play as a straight guy and sex — either you're Don Draper (who never talks about sex, only makes a move at the right time, etc.) or you're Mr. Bean.