I think it's important to point out that there are actually a few different reasons to be against every LI being bisexual. I agree with you that the argument that "It's unrealistic that every LI in DA2 was bisexual" is absurd. For me, I'm against the idea that every LI in every Bioware game should be bisexual, which is what a lot of people who are asking for this seem to want. "Why couldn't we just stick with the DA2 sexuality system?" comes up a lot, and I think that is an equally absurd statement. Unless the world building is that everyone is bi, in which case I'm all for it, I want my companions to be created with set sexualities. If they all happen to be bisexual, I have no issue with that. If they all happen to be gay, I have no issue with that. If all of them happens to be bisexual or gay or straight, but has no interest in my PC, I have no issue with that (well, okay, I'd cry a little).
The thing that sticks to me is that... According to Dragon Age lore, in Thedas, non-heterosexuality does not have the same attached social stigma that it does in our world. Fereldens find it unusual but are accepting, Orlais calls it a 'quirk of character,' Antivans encourage polyamory, the Chantry has no official stance... Only Tevinter seems to be outright against it, and even then, it's considered acceptable so long as it's kept to a favored slave and there's a public marriage to allow bloodline continuity. Basically, there's only pressure to have a relationship that results in a child to ensure the continuation of specific bloodlines, and otherwise, in Thedas, that stigma of non-heterosexuality is hardly there.
And in our world, history shows that when there isn't a social stigma attached to sexuality, acting on non-heterosexuality is much more common. So in that case, why WOULDN'T more of these characters be bisexual, and why would it HAVE to be considered a character trait? It feels to me that to say that these characters aren't at least open to the idea is more of elements of our world's societal stigmas intruding, rather than a stigma that is true to the world that has been built up in the games. To these characters, if they're not heterosexual, it wouldn't HAVE to be something they'd draw attention to or make a big deal out of, because their world DOESN'T make an issue of it.
Especially when I've seen the excuse of 'it didn't fit the character for them to be [x]' as to why characters aren't established as bi. Sexuality just says who a character will sleep with, nothing more. How does it fit or not fit a character? What is this checklist that a character needs to fit in order to 'fit' being bi? I'm not against it being a character trait, but when it is, it's often used more as a crutch - 'they're bi, so they'll sleep with EVERYONE, so they're going to be going around, flirting with anything that moves!' I don't want to see this anymore, because I see it as an unpleasant stereotype, that because this character is bi, they're going to be attracted to everyone and try to get in their pants.
My preference is that LIs be bi-unless-gay, since there are so few gay characters in media. Just... give the most content to the most players and let them pick their chosen romance from there, no problem. Though I'm not against there being characters with a gender preference, that you have to work a little harder to get them to enter a romance with a given gender PC. Like, say for example, if there's an approval system like in DAO/DA2, a character would respond to flirtations from opposite gender PCs at +25 and same gender PCs at +50.