Well, according to my understanding, it is not as complex as it may seem.
I mean, if you are a tgirl (assuming some degree of feminine presentation, mannerisms, appearance, etc) just try to go hook up with the muscle hunks at a gay bar. Lemme predict what would be your outcome: no one interested in you even if you begged for sex (no matter you're pre-op, your size, etc).
If any girl had success here please tell me where lol

Jokes aside, now go to a straight bar and see how it goes. Wildly different, right? You don't need even to approach the guys, am I right?
And now if you are post-op and tell or if you're pre-op and tell, then some guys (maybe many of them) just run away.
Isn't it simple? I mean, its obvious "they liked you right until the moment you told you're trans". Is it within you? Obviously no, is just in their heads. They saw a set of characteristics (physique, mannerisms, etc etc) which they liked a lot, but then, because of an idea (your "trans status") they just backed off. If you had not told, or if they hadn't saw "your thing", they would have just continued.