Probably a lot of different things, for example: revulsion at some of the traditional gender roles and behaviors. Negative emotions about their sexual organs. Intense erotic pleasure while imagining themselves the opposite sex. Anxiety due to not feeling what they think the person of their sex is supposed to feel.
Why do you think such a meme would spread or originate, if not due to its truth value?
Memes that provide an explanation of one's behavior in terms of one's identity are insanely powerful. They spread because they lead you from from "I don't understand why I'm like this" to "I understand why I'm like this", and the latter feeling is something we all lust for.
The truth value is not especially important to the initial spread of an attractive identity-meme. Consider that "people are born gay" is almost a dogma in the LGBT community and liberal circles, although the available scientific understanding sharply contradicts it. Or recall that the 19th century saw a very potent meme in which gay people self-identified as "the third sex", "a female psyche in a male body". It seems that many gay people in the 19th century really felt very strongly that they have a "female psyche" or a "female soul", similarly to how today many biological-X transgender people feel very strongly that they have a "non-X brain".
That historical example did a lot to persuade me. Do you have any others similar to it?
I used to share your position, but moved away from it. The main reason I did is studies such as the ones mentioned in this article:
http://online.wsj.com/articles/SB10001424052702304854804579234030532617704.
How do you explain such results?
What is something you used to believe, preferably something concrete with direct or implied predictions, that you now know was dead wrong. Was your belief rational given what you knew and could know back then, or was it irrational, and why?
Edit: I feel like some of these are getting a bit glib and political. Please try to explain what false assumptions or biases were underlying your beliefs - be introspective - this is LW after all.