I agree with you, that scene doesn't prove that he is 100% straight. But, my point is that there is no reason for him to be uncomfortable with his sexuality (if he is in fact bi) because people are comfortable with s/s relationships and they have been forever in Thedas as you pointed out in one of your posts.
I can see what you mean by this, but there is still some evidence of people being uncomfortable with s/s relationships. Both Gamlen and Oghren express some "ew, who's the girl?" dialogue with respect to s/s relationship. So it's not that there isn't discomfort with s/s relationships from some people, it's that there isn't an institutional homophobia being driven by the Chantry, like we see in the real world. So, it stands to reason that Cullen could still have some hesitation because of how other people might perceive him. And, it could be something that he, as a person, could just struggle with. We'll have to wait to see (if he's even bisexual) in DA: I how he addresses it.
I'd rather him be straight as it seems more in-line with Cullen as a character, but for the fact he's so popular and this is the chance we'll finally be able to romance him, Bioware might be afraid it'll look bad if males can't get with him. They did it with Kaiden and it appeased a lot of fans, if Cullen lost the chance to romance males there could be a lot of backlash to come from it even though I personally think he's straight.
Why does Cullen seem "straight" and not "bisexual" regarding his character? Outside of a single dialogue option in one out of the six origin stories (and then only when you play as a female), he doesn't give any indication whatsoever to his sexuality.