If someone doesn't like Dorian's quest because they don't think it was well-written, so be it. I'm not about to argue with them.
If one's objection, however, is that having a single follower quest -- out of the four followers who are gay or bisexual -- which makes their sexuality at all relevant somehow makes that the entire point of the quest or, indeed, Dorian's arc as a character...then I'm afraid I'll simply have to shrug and say their comment says more about them than anything else.
It's rather like the occasional messages I get from certain parties who complain about how the game "caters to gay people" ...why? Evidently because there isn't enough content for straight people (meaning more than they got, which is actually the same as in previous DA games), because they don't like the content they did get and ascribe that to either a gay or feminist agenda which is being "rammed down their throat" (an odd metaphor to select, if you ask me), or because those characters exist at all...and the mere existence of a few characters or themes that relate in any way to the actual world we live in is somehow too jarring a notion in a mature-themed game already full of themes that cover almost everything else under the sun.
It's an odd thing to obsess this much over, considering there's so much else in the game to occupy one's attention, but hey -- you do you.
Speaking personally, I do feel like Dorian's personal mission felt like a bit of "wasted potential" just because there were so many fascinating aspects to his character. I think more could have been done with him. Compare, say, Cassandra, who got a quest of "personal" relevance with Swords & Shields, as well as a "plot"-relevant personal quest with the Seekers. Granted, the Swords & Shields quest wasn't particularly heavy, so it's not a perfect comparison, but, hey, doesn't Dorian deserve better anyway?
That all said, the meeting between Dorian and his father was, in my opinion, so gosh darn well done in pretty much all terms, that I'm glad it was included. There was clearly effort put into it, and it definitely felt like an emotionally-charged scene about family than it did a "here's our token 'minority' moment to appeal to a specific audience" and I only came away from it loving the character more.
But still, guys, c'mon, that's all the more reason we need more content about Dorian and his homeland and the Venatori. Please? Guys? Please? Can't we please has "Dorian's Song"?