Im sorry, is this Jr. High? Are you seriously going to make that the quest that defines Dorian? A quest that basically boils down to "he is gay"?
I did not feel that way, actually. I had not read anything about Inquisition before playing it, or even before ending my first playthrough. I just experienced while I went along (a deliberate decision).
So, I only even realized that Dorian is gay when that came up. Alright, maybe I am a little stupid, but I did not feel he was a "steroetypical gay" and only this personal quest made my female inquisitor understand that all the previous flirting had been useless. :-) I liked that.
Anyhow, for me it was really many things - putting the cards down on the table, addressing Dorian's family issues, showing a glimpse of Tevinter society. It's about him being gay, but not only. It is also about non-conformity, living his own life, making sacrifices for that (sacrificing his family, in the end). And it is about loving parents who still cannot accept their children as they are, trying to change them even by means they used to despise. And it is about biting the bullet and saying "I'm sorry, I made a mistake". Showing that this powerful Tevinter magister ist just human after all, with the same sort of emotions, faults, and regrets... for me, this made it important, especially in the face of Dragon Age lore.