What do you think that something is that they feel?
Why do you think such a meme would spread or originate, if not due to its truth value?
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 do...
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.