No it wouldn't, it would have required re-animating all scenes with every gender and race of Inquisitor. And the romance lines are there already, the animation is missing. No it hasn't been exactly stated that being bi just didn't work for his character, Bioware actually hasn't even responded on him being bi in some part of development.
He called qun following qunari heathens and I really doubt Josie, Dorian etc. have been really seen many qunaris or dwarves before and been into them so I don't think these matter at all. I think it was purely because time issues since no reason for gating was found in the game although BW promised there would be story-related reasons.
Alright, I'll give you that one on the animation part. But they have responded on his sexuality.
See http://www.ladyinsan...terview-gaymerx
He basically says that he hated the "everyone is bisexual" thing in DA2, and that some characters have to be straight, some bi, etc. and that they eventually decided to make Cullen straight because it worked better with his character.
As far as the race-gating thing, you could be right, it is after all subject to opinion. But, some people are also more picky about what they are into than others, Cullen just might be one of those. Josie, being a diplomat, might be more willing to experience different cultures and races than someone like Cullen who lived a more sheltered life, and has also had bad experiences with say, Qunari.