From the link I posted:
What causes homosexual desire?
If homosexual impulses are not inherited, what kinds of influences do cause strong homosexual desires? No one answer is acceptable to all researchers in the field. Important factors, however, seem to fall into four categories. As with so many other odd sexual proclivities, males appear especially susceptible:
1. Homosexual experience:
any homosexual experience in childhood, especially if it is a first sexual experience or with an adult
any homosexual contact with an adult, particularly with a relative or authority figure (in a random survey, 5% of adult homosexuals vs 0.8% of heterosexuals reported childhood sexual involvements with elementary or secondary school teachers (5).
2. Family abnormality, including the following:
a dominant, possessive, or rejecting mother
an absent, distant, or rejecting father
a parent with homosexual proclivities, particularly one who molests a child of the same sex
a sibling with homosexual tendencies, particularly one who molests a brother or sister
the lack of a religious home environment
divorce, which often leads to sexual problems for both the children and the adults
parents who model unconventional sex roles
condoning homosexuality as a legitimate lifestyle? welcoming homosexuals (e.g., co-workers, friends) into the family circle
3. Unusual sexual experience, particularly in early childhood:
precocious or excessive masturbation
exposure to pornography in childhood
depersonalized sex (e.g., group sex, sex with animals)
or girls, sexual interaction with adult males
4. Cultural influences:
a visible and socially approved homosexual sub-culture that invites curiosity and encourages exploration
pro-homosexual sex education
openly homosexual authority figures, such as teachers (4% of Kinsey's and 4% of FRI's gays reported that their first homosexual experience was with a teacher)
societal and legal toleration of homosexual acts
depictions of homosexuality as normal and/or desirable behavior