I'd like to state now I think making Tevinter homophobic was a stupid move and diminishes my enjoyment of the game.
Sorry guys.
But there's no reason for it, IMHO, and makes the games lesser.
Tevinter is not homophobic... not exactly, at least not in the sense you are using the word here (if I understood correctly).
In my opinion, Tevinter is more on the "master race" side of things. They cultivate the notion that being a mage and of noble origin means you are superior to anyone who isn't both these things. They (aka nobles with magic) are in a place of privilege and power due to this, and like most people who are in such a position, they have a few priorities. First and foremost would be keeping said power/privilege. A close second I would venture to say would be gaining more power/privilege, but that is another discussion.
Back to the keeping of power/privilege, how do you do that? Well, obviously you want to maintain the status quo in society. Best way to do that is to ensure that you have descendants who are also mages so you can pass on what you gained/stole/etc. (a variation on the "nobility doesn't mix with others" type of concept)
Basically, what I'm saying is that they seem more concerned about protecting their own privilege (which means bloodlines, biological children, and more specifically bio kids with magic), then they are about how "wrong" being gay is. From that, it would be logical that a "it's deviant behavior" attitude would result, as a way to reinforce socially the necessity of perpetuating family bloddlines (to make sure you and your descendants share in on the power/privilege and don't end up as slave themselves because the magic trait died out in the family).
Hmm... a bit subtle as far as differences of opinions go, but yeah. That's what I got from Dorian's quest. I think it fits well and makes me despise the tevinter "evil" elite a bit more. Well, a lot more, homophobia tends to bring out a sort of viciousness in me lol.
But I think it can add a clear separation in terms of what is worth saving/reforming in Tevinter, vs what isn't or is beyond redemption. Afterall, the more issues, the more drama, the better the story and game
Tevinter's "evil" mages already have slavery going for them, now add this. They are turning out to be really really despicable. Though I'm sure there will be people who support those things lol (just see support for qun, in terms of... yeah... lol).
Eh, I digress... 