Birth defects
Most birth defects are not caused by defective genes. Most are caused by external toxins, lack of oxygen due to bungled birth, stress in the mother, starvation, etc.
In the case of "homosexuality" and perhaps all sexual aberations of a psychological kind, it is caused by low androgen levels in the mother - due to stress. With the lesbains - which are very rare it is caused by adrenal hyperplasia - which is very rare.