Born this way
The topic recently came up in an anthropological discussion, so I decided to put some meat to the bones.
http://rspb.royalsocietypublishing.org/content/271/1554/2217.short
A statistical proof that male homosexuality is genetically inherited and is related to increased fertility in female family-members.
http://www.sciencedirect.com/science/article/pii/S0301051107001433
A statistical proof: About a third of men and two thirds of women are somewhat bisexual and most likely the cause for homosexuality is genetic.
http://informahealthcare.com/doi/abs/10.1080/09513590400018231
Sexual orientation is determined before birth, depending on which hormones the developing brain of the fetus is subjected to.
Quote (page 8): "Prenatal nicotine exposure has masculinizing/defeminizing effects on sexual orientation of female offspring and increases the probability of lesbianism."