One of the primary points Zandilar is making, though (correct me if I'm wrong here, Zandilar) is that it's not just a matter of backstory but also of ongoing interaction. Where the character normally wouldn't respond to someone with interest because they're not interested in that gender they would be more likely to if that gender were suddenly their flavour, so to speak.
Making Kira into a bisexual woman would, for example, not just be a matter of changing how she reacts to the female PC but also, in theory, every significant female NPC in the game. That means a lot more in the way of dialogue changes.
That is, if I'm not wrong, one of Zandilar's main points and (I suspect) the main one that's getting missed.
Yes, this is exactly the point I was trying to make. Further, it's like chaos theory (or whatever, I'm paraphrasing the example here) - you know, if you go back in time and squash a butterfly that wouldn't otherwise have been squashed, everything from that point on changes (or if it simply flaps it's wings in a different direction... whatever). A lesbian Kira would have a different life path to a bisexual Kira or a straight Kira, simply because in the past, some of her interactions with other people would be different (flirting with someone she otherwise wouldn't have flirted with) and would therefore have changed the way some events panned out (aforementioned flirting might lead to a relationship that might not have happened), leading to very different characters (or not - because it is also possible that not much would change... but it's not realistic to expect that "not much would change" would be the outcome for every LI with a discrete sexuality).
Kira's potential past flirtation and relationship might not ever be mentioned in the character arc as presented in the game, but it would have an effect, nonetheless, as part of her background.