Wow, that is a big question!  Unfortunately, it is unlikely to be addressed in any real scientific way.  We do know from twins raised apart that genetics can account for about 50% of sexual orientation, the remainder is likely to be environmental, including the fetal environment. 

So does the environment influence sexual orientation through epigenetic mechanisms?  Really difficult to answer because we do not know where to look.  Epigenetic modifications are tissue and cell type specific.  Should we look in the brain, where morphological differences have been described?  Even if we could get enough tissue samples from homosexuals and heterosexuals, their epigenetic imprints may be very individualized, i.e. we have no normalized controls.  It is an interesting idea, but unlikely to be studied in any real meaningful way in the human population.

Thank you Gregory for your answer. I see your point about epigenetics. I did find numerous research on genetics, prenatal development and some on infancy and childhood factors (although those seem politically incorrect), but none on the compounding effect of more than one factor.

Thanks again.

where morphological differences have been described?  Even if we could get enough tissue samples from homosexuals and heterosexuals, their epigenetic imprints may be very individualized, i.e. we have no normalized controls.