A companion with a backstory involving an old boyfriend could as easily have had an old girlfriend, and only that single term need be adjusted.
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.