As long as we are fighting oppression, the LGB and the T are natural allies, so grouping us all together makes sense. In fact, it is not well known that, in the Stonewall riots that began the "official" struggle for gay rights, trans people were at the centre of it. It would have been just another routine police raid if they hadn't arrested the cross-dressers, provoking the riot.
Yes, as logical categories, sexual orientation and gender identity are separate things. But the political and religious opposition to our existence is much the same. Considering that the LGB are about 30 years ahead of the T in fighting back, we could learn a lot from them.
And there is the fact that most of us are gay or lesbian either before transition or after, or else bi throughout. So I think we have quite a bit in common.
The alphabet soup gets a bit ridiculous, though. Now it's up to LGBTQIA2+. For sanity, it's pronounced "queer".