Here is an argument about an important difference between being gay and being trans.
-- It turns on the fact that, for some X, but not all, to feel that one is X is just to be X, and consequently, while some self-descriptions are self-validating, others are self-delusive.
Suppose I feel that I am angry. It will follow that I am angry. Feeling angry is being angry.
However, suppose I feel that I am fat. It will not follow that I am fat. Feeling fat is not being fat.
( Note , in passing, that feeling angry does not explain or define what it is to be angry. We know what it is to be angry independently of the equivalence of feeling angry with being angry. The equivalence, to make any sense, requires this independence.)
OK, now, in respect of this 'feeling/being' equivalence, being gay is like being angry; being trans is like being fat.
Why? -- Look (suppose, for definiteness, I am a man):
To say I am gay is to say I feel that I am attracted to men. And feeling that I am so attracted is being so attracted. So if I sincerely say I am gay, I must be gay.
However, to say I am trans is to say I feel that I am a woman. And feeling that I am a woman is not being a woman. No matter how sincere my avowal, my feeling so will not guarantee my being so.
(Here we might need the note from above: we need independent knowledge of what it is to be a woman, in any case.)
So being gay is wholly different from being trans. A sincere self-description of one's sexual orientation is necessarily veridical, whereas a sincere self-description of one's sex having changed is mistaken.
[This argument may be partially evaded by denying that a trans person changes sex. But that is OK, I suppose.]