You cannot say "they are not doing [sexuality] the way I think you should do [sexuality] and therefore I will accuse them of pretending", because you don't know what they're thinking, you don't know how they're reacting, and you have no way of knowing what they consider themself short of asking.

Nicely put. I think people pretend to know too much, too much of the time. Relatedly, I wish people would remember that in real life you only know isolated facts about other people's behavior, not platonic truths. The polite thing to do is never to contest someone else's self-identification as to their sexuality. But you don't actually know anything. Maybe they are doing what they are doing, or presenting their public face, or whatever, out of fear, or pressure, or inertia, or lack of opportunity. You can't know for certain.

So I try to never claim anyone is straight or gay or anything, just what they self-identify as, if I know that, and who I have seen them do stuff with, if I think gossiping is ethical. I don't always succeed, but I at least aspire to sticking to observable facts.

Another aspect of this, for people who think labels are bad and no one should use them to describe themselves or others, ever, is that you can avoid using them without making a big, obvious deal of it.

Personally, I don't think labeling our sexuality is bad, per se, but I do think it is a symptom of our species taking too long to evolve past primitive tribalism. If aliens decided to make contact with our species now, I'd be horribly embarrassed by a lot of things. In the meantime the labels themselves are not a big deal.