It's common sense and a little bit of biology. Humans cannot procreate if they are committing homosexuality. Therefore, you cannot be born gay (since it'd have to come from your genetic makeup - since both of your parents would have to be heterosexual for you to have been born in the first place, there is zero possibility of you obtaining a gay gene from them - even if your genetics were to skip a generation, homosexuality PREVENTS passing on the gene in the first place) Hence you cannot be born gay.