It may very well be a congenital birth defect caused by a lack of nutrients and minerals in the mother during pregnancy.
I recently watched a true crime show on TV where a lesbian “flipped” a straight woman and they became “lovers”. I think it is just a case of arrested development and these people never matured past the “playing doctor” phase of childhood. It is simply mutual masturbation. But that is just this old lady’s opinion.
The strongest theory, IMHO, is that the primary cause of homosexuality is narcissistic parents.