Vivienne would have to be toned down if she was a romance option, right? I mean, as it is, performing cunnilingus on her would probably lead to my tongue freezing solid, and then I'd probably get cancer somehow.
Personally, Vivienne's character is interesting because people like to see her next Tuesday. It's like Morrigan, and I doubt people who are a fan of Morrigan wants her to be any less selfish. Vivienne is basically a karma Houdini, she basically gets away with everything in this game, being a mage and enjoy privilege and royal life of a normal person, you can punch Dorian and Solas at low reputation but Vivienne basically shows you off, rearranging your furniture, lecture you and you can't do anything for her, she basically make snide comments about mages and attack all the companions during the dialogue and win most of those arguments, and could even become a divine. It's funny how the world is basically bend to her will. I guess that is her character.
Still, the point is that outside of Vivienne, there isn't any possible option, and Bioware didn't even consider Scout Harding as a possibility, I don't think. I guess I could understand why both of those options were males since there aren't a lot of females in this game.