Articulatory Loop

Yet another study has found male homosexuality to be linked to the X chromosome, but this time with an extra interesting twist: whatever gene or factor it is also confers increased fertility to the mother. They found that female relatives on the mother's side of gay men had more children on average than the female relatives of straight men.

Bear in mind: a genetic link does not mean that being gay is 100% genetic. It almost certainly isn't. But it is probably something that's built in to genes, meaning they have the capacity for it, but need environmental factors to activate them. These could be gestational environment (hormone levels, stress, nutrition, infection), they could be parenting styles, home environment, exposure to radioactive waste, whatever.

You could count on two hands the number of traits that are purely, 100% due to "nature" or "nurture," independent of the other.
Off the top of my head:
100% Nature -
Hair color (without dye)
Huntington's Disease
Cystic fibrosis
Tay Sachs
Color blindness

100% Nurture -
Religion (which one)
Language (which one)
Political affiliation