@Lebanese Dude: Question: say if Cullen only ever expressed an interest in a male Warden in Origins and was only able to be romanced by a male Inquisitor in Inquisition, yet never explicitly stated that he was gay, would you still be insisting that he could be potentially be bisexual? I'm genuinely curious here, because I just can't understand the mentality of some people on this forum. I mean, come on, Bioware is a fairly progressive company - if they wanted Cullen to be bi they would have made him available to a male Inquisitor as well, but they didn't, so it's obvious that he's straight. You'd have to be fairly dense not to realise this. Does it really need to be spelt out for you?
Take Juhani from KOTOR as another example of what I'm trying to say. She doesn't explicitly state that she's a lesbian, but it's fairly obvious that she is when you consider that: 1) She only ever expresses a romantic interest in you if your character is female, 2): it's implied that she was in a relationship with a female Jedi. You don't need to be Sherlock Holmes in order to put the pieces together here. You just need to use your brain ever so slightly.
However, if we go by your logic, she could be bisexual as she never explicitly stated that she's a lesbian. Sure, there's a possibility, but it would be rather unlikely, don't you think? I do. She's a lesbian, I don't need her to come out (pun unintended) and tell me that she's a lesbian. I've already figured it out - I'm not that stupid.
Yes, you can use common sense to figure out someone's sexuality most of the time. But, if you make assumptions about people (and we all do it) based on what you observe about a person, you might be wrong. That's the point. There's really nothing in the first two games that requires a retcon for Kaidan to be bisexual. He had a crush on a girl back in high school and he's willing to break fraternization polices for FemShep (but not mShep) in ME. Okay, he could be straight. He's not gay. But there's nothing there to say that he's not bisexual. That's the point.
With your scenario about Cullen, yes, I would think that Cullen would possibly be bisexual. Just like, technically, Steve could be bisexual. I don't think he ever identifies as gay. He just had a husband and was interested in mShep. Samantha, on the other hand, is not bisexual because she identifies as lesbian (if I remember correctly). Dorian and Sera? Both identify as gay. If they were bisexual in the next game, then it would be a retcon. Juhani, technically, could still be bisexual. So could almost all of the 'straight' characters because they rarely turn down a s/s PC. It's usually done by just not offering flirt options at all. If they offered them and then the person said, "Nope, not interested" to only s/s PCs, then I tend to think that this is enough evidence to say that they are straight. It's why LebDude and I disagree about Cullen in this case. I think the fact that he can flirt with a male PC and then turn him down is enough for me to see that he's straight. It could be clearer if he said specifically that it's because my character is a guy, but I don't think that's necessary.
Does this make sense?