Ferelden and the other nations don't because they are the good guys and you are supposed to be rooting for them so they are shown to have more progressive attitudes to women and gay people and whatnot. Tevinter are the bad guys so they need to represent everything you hate like slavery, puppy killing and conservative views on things like homosexuality in order to make it clear who the good guys are and who the bad guys are.
This is what the World of Thedas has to say on the subject (p. 72):
The view on indulging lusts with a member of the same gender varies from land to land. In Orlais, it is considered a quirk of character and nothing more. In Ferelden, it is a matter of scandal if done indiscreetly but otherwise nothing noteworthy. In Tevinter, it is considered selfish and deviant behavior among nobles, but actively encouraged with favored slaves. Nowhere is it forbidden, and sex of any kind is only considered worthy of judgment when taken to awful excess or performed in the public eye.
I don't think likening Tevinter attitudes towards sexuality to modern conservatives is exactly accurate.
Edit: now my post is redundant because someone said the exact thing as I was typing hehe.