A recently released Danish study suggests that early childhood and family factors could have an influence on sexual orientation.
In general, early father or mother absenteeism may be one influence. Others could be urban vs. rural habitation and birth order.
("Childhood Family Correlates of Heterosexual and Homosexual Marriages: A National Cohort Study of Two Million Danes," by Morten Frisch and Anders Hviid, Archives of Sexual Behavior Oct 13, 2006. - H/T Lifesite.)
I wonder what would happen in a same-sex family where one partner leaves the family early on in the child's life? I suppose that we'll have to wait for more Danish studies to find out.