It seems like it's a moot point now that there are no questions tagged lgbt. And not being an active user here, I don't want to try to say what tags should and shouldn't exist.
But I'd urge you not to regard lgbt and sexuality as synonymous, or to treat lgbt as a subset of sexuality.
sexuality includes straight sexuality, and clearly focuses on the sexual aspect of things. That is, it's about sex, not sexual orientation. This is demonstrated both by the tag wiki excerpt ("Sexuality is the quality of being sexual or possessing sex.") and the questions so tagged.
lgbt doesn't really have to be about sex at all. Yes, the LGB part is sexual orientation, but plenty of LGB issues are really nothing to do with sex. And the T is about gender identity, not sexuality. (While it obviously has some connections to sexuality it's clearly not just about that.) In any case, questions can be about LGBT topics without being about sex.
So consider this: would you apply sexuality to all questions about straight relationships, e.g. everything that mentions marriage? I don't think so. It doesn't seem any more appropriate to apply sexuality to all questions about LGBT topics. If anything, it might be a disservice to the important, meaningful topics of those questions. "What are Wizarding society's attitudes towards homosexuality?" is really not about the same subject as "How many alien women has Capt/Admiral Kirk slept with?"