Science don't have a real answer to the question why and when someone become gay. But I don't think that more ppl will become gay because they have same sex parents.
I mean most gay ppl were raised in heterosexual families 
Some people are born homosexual, others are turned into Homosexuality due to specific circumstances (like sexual abuse in childhood). I think it's possible for a child to believe that homosexuality is a very "normal" thing, if he/she was raised in a homosexual family. He/She might grow up bisexual, but also he/she might change his/her "attitude" because of what he/she was raised to see.
Oh by the way, science might have an answer why someone becomes gay:
http://www.livescience.com/50058-being-gay-not-a-choice.htmlwww.sciencemag.org/.../homosexuality-may-be-caused-chemical-modifications-dna