However, that said, Dorian does say "there will always be an us" and "you are the man I love", which is near enough a wedding vow, so who needs an official ceremony? Have to admit just a tinge of jealousy for Sera though. When she proposes the Inquisitor can say that marriage is all about titles and inheritance but Sera dismisses that idea because marriage can mean what they want it to. Totally agree. Still I have an elven Inquisitor, so he wouldn't recognise a Chantry sanctioned ceremony anyway. He's bonded for life in the traditions of his people and in my head canon exchanged simple vows with Dorian according to his own customs. Come to think of it, I don't suppose Dorian would recognise a southern Chantry sanctioned ceremony either. If there is a Maker watching over us, as Dorian believes, then he would be aware of our commitment to one another.
He's not a fan of the Chantry as an institution, whether that is in the North or South, but people tend to have very specific ideas when it comes to marriage. Someone who was raised in a faith, but not a practicing whatever, might still feel that marriage is a huge step in their life, so they want to go all the way with a religious ceremony and such. You never know how people are about these things until you discuss it.
As for Dorian specifically, I'm not sure how he would feel about marriage in general. I wouldn't be surprised if the concept weren't tainted for him because of his parents' marriage, his own experience in trying to avoid it being forced on him, and just the general meaning of marriage for nobility. However, I can see him accepting a gesture of commitment. I'm not quite sure if he would make one unprompted himself, though. He just might not think that is something he needs to do, or they need to do. Or... you know Dorian, he might feel that the Inquisitor might balk and not make one because of that.
And something I can actually see him saying if the subject were to be brought up, "That's not what men like us do, is it?" Because as a homosexual, and one who grew up in Tevinter, he would NEVER have considered being married to another man. It just wouldn't have been a concept that he even entertained in his mind. Being in a relationship was already a big deal. So, he might be confused, or disbelieving. I don't think he would react with immediate happiness.
It's tough to say, really.