You make a lot of good points, but this just isn't true.

The LGBT community doesn't consist of every L, G, B, and T person, however. Instead, it consists of everyone who identifies as part of the community, and it's very welcoming to most LGBT people, so it will indeed provisionally assign membership to you simply because you're L, G, B, or T (in the fuzzy unofficial way that group sentiments "assign" things).

So not only does a core community exist (due to community-building by a subset of people who identify with that community), but any random LGBT person has presumptive membership.

That's the thing about identity groups. Identifying with them is enough to make them socially real. If I identify as a Pastafarian, then as long as there are other devotees of the Flying Spaghetti Monster, it's a real identity group.

The point is, however, that within every basically every group, there is so much diversity that you should get to know individuals as individuals, not assign your perception of the group-average to them. Firstly, they probably aren't the average, so you'll be wrong; secondly, your perception is probably wrong, so you'll be doubly wrong.

As with almost everything, there are exceptions--where there really is a commonality that is worth paying attention to because it's so common. But you have to be ready to make exceptions to those exceptions.