where does gayness come from?

personally I think its a nurture thing rather than a nature thing. people are not born gay but something in their life...a psychological trauma for example, think rejection..causes the shift in the mind. people are not born gay and if they were there would be no way to know! I dont buy the they have different brains from birth arguement for that reason, gayness is something which you develop surely? Like dare I say it... a condition/illness of sorts!?