You know, I'm bisexual, and when I initially started thinking on what I think the DA2 companions' sexualities are, I also didn't find the idea of all four of them being bisexual very believable. Because yeah, four people within the same group of friends who just happen, all of them, to have the potential to be attracted to the same person, as well as the likelihood of their feelings being returned, and all of these potential relationships turning out successfully, and all of them just happening to be attracted to that same person regardless of gender? Let's be honest: that's possible, but not really all that likely.
And then, I realized something incredible and mind-blowing! it doesn't matter! Because that goes for everything else in the game too. Hell, the fact that they're all potentially attracted/romantically interested in Hawke regardless of Hawke's personality or appearance is pretty unlikely too. So is the fact that they just happened to even know or meet Hawke the way they did. Or half the events of Dragon Age 2 aren't, realistically, super likely and are basically coincidences. And not nearly as much as Dragon Age: Origins. I mean, its plot basically only occurred the way it did and is based on a bunch of contrived coincidences. I mean, isn't the whole basis of fiction contrived coincidences? Events just happening to occur and unfold in the exact way the author dictates? Why cross the line at bisexuality? Why is that where we suddenly start caring?
Personally, I care much more about being given more than one option on who I chose to have my character enter a relationship with, not being forced into an opposite-sex option just because I want to romance a certain character, being able to play as a character who is like me without any fear of being judged or feeling uncomfortable about it, and having my existence acknowledged to one degree or another than any vague concept of 'realism'. And about not having bisexuality treated as some kind of quirk of character but as a perfectly normal, valid sexual orientation that all sorts of people have, because bisexuals are-- who would have thought!- people, who are just as varied as other groups of people!
And see, this is why I see all the four LIs in DA2 as bisexual. I like to think of them that way because it's nice, for once, to have in fiction four totally different people, all of them with different personalities, characters, who are equally complicated and deep characters, that also happen to all be bisexual.