As much as I am disappointed over the Cass reveal I'm pro set sexuality in this game too. I hated the route they went with DA2, glad they went back to the DAO model. I prefer that the LI's have their own preferences, makes them feel more real in a sense, like us we all have different orientations. And if it fits the characters okay fine, I'll just have to work around it even though some of the choices weren't ideal for my preferred gender playthroughs.
^ This.
I was terribly disappointed by Cass' sexuality reveal as well as Cullen's, (despite it not directly affecting me), and I may still be by Vivienne's if I'm lucky enough to even get a romanceable Vivienne, but I do truly believe that the set sexuality route is the best route, regardless of the immense heart ache it might cause.
As Gaider so eloquently put it:
'DA2’s setup was a compromise, but not a great one as bisexuality itself is not a compromise. It is not “all of the above”, but rather a specific choice, and having four bisexual characters is not true representation.'
I'll take the true representation any day. And hope Vivienne is part of that.