I don't really see the issue, tbh. Well, I do see the issue, but I don't personally align with it.
I'm far more interested in seeing two compatible personalities interact in fiction, and gender doesn't generally contribute to that dynamic, unless you're using their sexuality as a plot point to drive the narrative. I'm not a fan of that, generally, but it can be done well.
I'm all for set sexualities in fiction, and representation is a force for good if handled well (like anything), but I think there should be some allowances within the fan sphere. Dorian's character and Vivienne's character could make for a fascinating clash of cultures and ideals, what with the free mage of Tevinter contrasting with the restrained control of southern Thedas, and I wouldn't see the point in restricting that exploration in fanfiction, just as I wouldn't see the point in denying someone seeing something similar between Cassandra and Leliana.
That being said, it's probably a bad example, since Dorian's sexuality is so tied to his personality, but you get the general message. As long as it's not out of any malice, where's the harm?