I'm not surprised one bit. What Gaider usually considers canon is not anything I would choose personally so this is just another item I can add to that list. I don't think anyone should avoid experiencing the romances based on someone else's head canon, writer or not. If we all avoided relationships IRL based on the possibility that our partners would be better off with someone else then we would never have relationships. It would just be a world full of people avoiding falling in love on some preconceived notion that the other person would be happier with someone else. Not a very healthy outlook.
But it does suck to think the only two M/M options could have been more into each other than with you (if you see it that way based on Gaider's opinion and the Movember ads, Trespasser DLC et al) which could be the place some people are coming from.
If they are trying, in some way, to make Dorian and Bull canon then the least they could have done was make Cullen a M/M option though. My IQ would have been all over that blondie. 
Actually, I might start my Gay Cullen run soon. I've been thinking of it for a while now but I've been deep into TW3 and Fallout for a while and now I'm coming up for air and ready to play something else for a bit.
I come from a family littered with divorces and infidelity. Right now my sister abandoned her husband of 12 years and two children to be with his best friend, who she "always felt [she] had better chemistry with". I can assure you if someone would be happier with another person (especially someone that they already know and are friendly with), they will eventually end up with that other person if the other person will have them and dump you. I'd do it!
The thing is, what are we to interpret from the content with Dorian/Iron Bull vs. Dorian/Inquisitor. Do they appear to have a more exciting, more sexually fulfilling, and happier (reuniting often) together in their future relationship than they do with our inquisitor based on what we see; the extra romance content (slides) and recognition they are getting as a couple? This isn't a- maybe there is someone out there my LI or my character would be happier with that we haven't met yet- so it's unknown (I assume this is what you meant and that most certainly is always the case, that there could be someone out there better for us we haven't met). But here we have three individuals (Dorian, Iron Bull, and inquisitor) who are well known to each other, and two of the three individuals (Dorian/Iron Bull) are in the end getting extra romance content/companion dialogue/acknowledgment/a greater depiction of their love and its conflicts and their eventual overcoming it and resulting happiness in slides (we do not get, but something rather bland in comparison) as well as writer recognition, than the other couple seems, who's to say that given a chance they'd choose each other over our character. That's the icky feeling I get with these types of pairings. I can only go with what the writer is telling me or portraying to me in the story. And what I get is Dorian/Iron Bull romance is extraordinary (they overcome so much conflict and yet are happy together, meeting in secret) and Dorian/Inquisitor is ordinary (they are working on it from a distance).
Many are feeling this negativity for a reason. It's just some of us cannot articulate it well (like me^ for example) without seeming pathetically jealous and petty. But we have evolved these types of feelings to protect us from situations that may not turn out well for us in the end. I can't tell you how many times I have said "I got a bad feeling about this", but couldn't explain why. And heeding that "bad feeling" ended up saving me a lot of trouble.
For example, I have a "bad feeling" I should not have written this, but I am going to ignore it and see what happens. 
Edit: To me it is just the implication that comes from them getting more end game romance content and recognition as a couple (and as the only M/M romances to boot). Liara, for example, got more romance content than any other LI, and I lost count of how many hate threads have been made about her and the writer's for trying to force her (how it is interpreted by many) as our canon romance, even though nothing has been suggested that she is the canon romance, it feels heavily implied though. The Liara hate is so palatable partially because she get's so much romance content over other LIs that some go low-EMS ending to kill her off.