I don't see it that way. I don't think conversations about former lovers is that important in companion's characterization, if at all. How those became so so important that the lack of them automatically warrants them the califications of "bad writing" ? No offense, but this just strike me as the player's need of "proof" for labels they want to stamp on companions. For me, those labels aren't needed, at all.
I think you missed the point. There is QUITE a difference between a character that merely reflects the PCs sexuality, thus their (non-discrete) sexuality plays no role in their personality or background, and a character that has their own sexuality (which may or may not be explicitly stated), which has an influence on their background and personality.
Am I talking about labels? No, because labels are subjective and may not actually reflect a person's sexuality accurately. Nevertheless, people have sexualities (and yes, asexual is a sexuality), even if they can't be described in a simple/single word, and that sexuality influences just about every interaction with other people (regardless of gender), whether or not they're aware of it.