I sort of agree. I am a straight male but I'll explain how I feel about the portrayal of sexuality.
I am a big sports fan, right? Some of you here surely like your sports too and maybe have seen the media reactions when a member of a popular sport publicly outs themselves as bisexual or gay. Its a HUGE deal for the media and that annoys me because I just think "So what? Why do they have to make this a big thing? Can't they just be who they are without being a figurehead for homosexuality in sport?". I suppose what I am saying is it frustrates me how its still such a big deal when somebody is found to actually not be heterosexual. It becomes a defining thing and anybody famous in that position seems to get new fans just because of it, and constantly asked things about it. They are 100% treated differently because of their sexuality - and most of it is a 'positive energy' and good reaction from people trying to be nice but it does become the biggest defining trait about the person, it appears.
Relating that to Dragon Age, I think for anybody who sees the media or watches many tv shows they see that most of the time, being gay becomes a defining trait and it seems mostly like this in the Dragon Age games too. I agree it shouldn't be this way and it would be nice for them to change things with how they write gay characters.
Having said this, I do not support all characters being bisexual like a few posts have said. I think its okay the way it is with having a few straight, gay, bi as well as race-locked romances. The mixture does add a realism to the game for me, I just would prefer, as the OP said, to see a promiscuous straight character, a 'charming prince' gay character or more 'butch' lesbian. This would only serve to add a bit more depth to the game I think. I would be happy if they kept maybe one gay or bisexual character that conformed to stereotype, but for each one that does... have one who doesn't! Probably the best to do it.