The "pro-slavery" nonsense smacks of veiled homophobia to me. I mean really, the one tiny detail gets blown out of proportions, to attack the gay guy (I'm sure most will deny this while convincingly outraged, but it's so damn similar to the oh so familiar "we're trying to protect the children, we're not homophobic" argument/mentality, that I cannot be anything other than suspicious).
I'm not a "hater," (I like Dorian) but I am a gay guy and I don't think this is a particularly compelling defense of Dorian. His defense of slavery is outrageous and I wouldn't really consider it a "tiny detail." (btw, is it a tiny detail when Morrigan disapproves of CE Wardens not sacrificing their father in a blood magic ritual? perhaps, but I understand holding that against her) Dorian's opinions on slavery are realistic, and I like that he has them, but I don't think they should be minimized.
1. While in Tevinter, where slavery is the status quo, and being raised in a powerful, upper class, family, he failed to really notice slavery as being something horrible. The why for this is so ridiculously obvious it's not even worth mentioning, but let's anyway, because reasons. He didn't understand what slavery is, on a profound level, and how wrong it is, because he was a child. His parents raised him to believe it's normal, and like any child, he believed* his parents (though even phrasing it like that, it's less then it actually is). Also his struggle with being gay and not accepted would have made other issues even harder to deal with.
There's a difference between something making sense and being understandable, and being forgivable. Everyone's a product of their environment and experiences, that's true. I think Dorian's position on slavery is realistic. Corypheus's position that he should be the god-king of all Thedas is also realistic and probably rooted in a similar kind of privileged solipsism. I'm not saying Dorian's as evil as Corypheus; I don't know why I brought that up. There's a darkness within me. But isn't it fair to hold grown-ass adults accountable for their opinions and actions? I'd say it is.
And is being a minority really an excuse to be even more self-absorbed and unable to empathize with others? TBH I can't imagine what I'd be like if I were straight - even more a monster for sure, though. My politics now are already bad enough.
2. After he left Tevinter and saw how things are done in the south, he is willing to have a change of heart. He still has remnants of the old ideas from back home (the comparisson between slaves and people in alienages), thus clearly he still hasn't grasped the concept, but, and this is actually the important bit, unlike IB (and his idiotic argument he presents to Solas), he's clearly ready to learn and willing to change his opinion.
If you think Bull's argument with Solas is idiotic, (and I agree) have you seen Dorian's spat with the former Tevinter slave in Bull's quest? It's brief, but pretty embarrassing on his part because he's defending the indefensible to someone who actually experienced it. Both Bull and Dorian come to the party with some naive/highly objectionable politics which can change a bit. It's understandable that some people will dislike them for this, though.
I do agree that the Qunari are worse than the Imperium, (in many, many ways) but if someone's upbringing is much of a defense for them, then Bull's nonsense is much more forgivable than Dorian's. Because like Dorian says, at least Tevinter allows some measure of freedom of thought.
side note:
it comes across as a subtle form of bashing because the authors' tend to make Dorian look as bad as possible so they can make Fenris/Hawke/the Inquisitor/whoever-they-like-more look better.
When it comes to slavery, yes, people have every right to make Fenris look better on the issue than Dorian. He was an actual Tevinter slave!