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”.
The list you mention is not very strong evidence that “people are born with their sexuality”. It’s a list of correlations of varying quality and effect size that is subject to strong publication bias. More importantly, all of these correlations are perfectly compatible with the possibility that genetic/prenatal factors only partially influence one’s sexual orientation rather than completely determine it.
Please read the section on twin studies that opens the Wiki page you referenced. The epidemiological twin studies are probably the strongest evidence we currently have, and they suggest that genetic factors play a role but do not determine sexual orientation.