My point of View: I think that as you get older you make the choice of dating girls or dating guys. I think that there are a lot of different factors that can make you "become" gay. Some factors that I think that have a influence on how your life is played out when it comes to heterosexuality and homosexuality are whether you grow up in a family that is accepting of anything, missing a parent or both parents or if you were made fun of a lot in social situations. I think that neglect, lack of social events and missing a father figure or mother figure has a huge impact on your sexuality. I may be wrong but I feel that you have a choice and are not born "gay".
My friends point of view: He thinks that when you are born you are instantly gay or staright. Think about it, down syndrome is cause by a missing chromosome so it is possible to be gentically gay or straight. Research even shows this and it is apparent growing up that people even in there early adulthood are attracted to the same sex. Could research potentially determine desired sexuality at birth? I think eventually this can happen because there is so much undiscovered or unknown about the brain and how all of its functions.
Another thing to think about: Can animals be gay? I think my male dog is! He likes to lick other dogs penis.