There is more evidence to support that people are born gay, than there is evidence to support that they are not.
I would say, rather, that there is a lot of evidence supporting the idea that sexual orientation -- homo or hetero -- tends to develop long before the beginning of puberty. But whether children are literally "born gay", or whether they "become this way" very early in childhood, is an altogether different question.

And I would point out, by the way, that the acquisition of one's native language -- English, Russian, Chinese, or whatever -- is not the result of genes, nor of fetal hormones; it's something learned very early in childhood. Yet the fact that one's native language is learned from the childhood environment does not make it unlearnable or "correctable."

I'm a native English speaker, and even if someday I miraculously obtain "near-native" ability in Russian (стучать по дереву!) I will still be a native English speaker -- the grammar of English will always be more familiar to me. (And, under torture, I would say "f*cking Chr*st!" or "I want my mommy!", depending on the degree of pain -- but I'd scream it in English, not Russian!)

It's also possible, in theory, that a homosexual can learn to enjoy sexual activity with the opposite sex, and thus become "more bisexual" rather than strictly homosexual. But that doesn't mean they lose their same-sex attractions and become heterosexual. (I think that people can probably "shift a bit" on the Kinsey Scale, but there's little evidence that people can move from the extreme homosexual end to the extreme heterosexual end, or vice versa.)

As far as I know, there's only one SURE way for a gay person to become an "ex-homosexual," or for a straight person to become "ex-heterosexual," or for a native English speaker to become an "ex-native-English-speaker" -- горбатого могила исправит!