> If I knew someone casually for a while who never talked about his/her sexuality, but one day they said "I never said this before, but I'm a heterosexual", I'd certainly feel awkward about it. Why did he/she say that to me?

That's because the dominant mode of society in _most_ cultures involves a heterosexual identity; it's just so deeply embedded in culture that most people can't tell. For the longest time, it was (and still is in many cultures) socially acceptable for men to ogle women. Why? Because there was a tacit understanding that part of being a man meant being hypersexual, and because all men are attracted to women (in this trope), ogling women is just part of "being a man". This ties a man's behavior and identity with his sexuality.