It's possibility as well. However Cullen was extra romance added in the game during last year of development and his race-gating seemed also be merely because of time issues as well. Also it seems like he was thought of being bi quite much since voice files even involve other characters like Dorian commenting on male Inquisitors relationship with Cullen.
i do remember that Gaider (I think) mentioned at GaymerX that there was a character that was going to be bisexual, but it just wasn't working for the character. I don't think that they elaborated on it; it was in the panel where they discussed creating LGBT characters, I think. I don't have the exact link, though.
If that's the case, I suspect that it was Cullen who they were referring to and that it had something to do with the fact that he wasn't interested in a male Amell/Surana. That's my gut on it (which might be completely wrong
).
With regard to Cullen, I was disappointed when he was announced as straight, but I had suspected it given that he was only interested in female Amell/Surana in DA: O and, for some reason, the DA team has made all of the male knight in shining armor types straight. When I found out that he was written as bisexual in DA: I and that content was cut, I was really, really disappointed because he's the exact type of romance that I have been waiting for through many games.
But as I thought about it, I'm okay with it. Obviously, I trust the writers with their characters. If they say that it wasn't working for his story, then I support them. The fact that they were thinking about making a m/m knight in shining armor type in this game gives me hope that we'll get one in the next game. With regard to patching in or modding Cullen's sexuality, I'm not in favor of it. The character has been created and he is what he is. I'd rather they focus on fixing bugged content and creating awesome DLCs. And, of course, creating an awesome DA 4.