I have spent alot of time aaround people who are gay and lesbian, and bisexual. One thing i noticed is that every single gay or lesbian person I have spent time with has been sexually molested in childhood. This is totally anecdotal of course, but just an observation.
Childhood abuse is a very large factor.
Male homosexuals I have met overwhelmingly have had one or more of the following in their lives/past. (now this does not mean that if you have had any of these things happen to you, that you are gay, just stating that these items are highly common in people who are gay.
Molestation/sexual abuse
Bad or no relationship with their father or father figure.
Overbearing mother.