Right. Even granted that it's true that orientation is inborn, that doesn't mean that reducing it to a few distinct, clearly labeled categories is any less of a social construct. In reality, it's a continuum, and individuals can occupy not just a single point on that continuum but a range of behaviors within it. So any attempt to draw clear dividing lines between categories is something of a fiction. It's like breaking the wide range of complexions down into "white" and "black," or hair colors into "blonde" and "brunette." Where's the dividing line? Someone who's considered "brunette" may have lighter hair in the summer than someone who's considered "blonde" has in the winter.
As David Gerrold wrote in The World of Star Trek about category labels like white, black, male, female, young, old, etc., these words describe people rather than defining them. In our culture, with our preoccupation with sexual preference, we tend to treat it as a major defining factor of identity. But once those prejudices and hangups are defeated, people might not make such a big deal over the differences between one individual's sexual preference and another's. It wouldn't necessarily be seen as a key factor in identity, merely a matter of individual taste, something that's expected to vary from person to person and thus isn't made a big deal out of.
Which, I should make clear, has nothing to do with whether it's inborn or chosen. Differences in individual taste in food can be a matter of innate sensitivities, allergies, things like that which aren't a matter of choice at all. But we don't define people's entire identity on the basis of their tastes in food or drink. We just accept that those tastes differ. We treat it as an individual variation, not a set of clearly labeled identity groupings.