"Show me the heterosexual gene."
Funny thing about genetics....no one knows what each gene strand does what yet. They can't even tell you whether a person will be born blue eyed or brown eyed.
Edit: As to the evidence bit, there are over 1000+ species on this planet that have homosexuality members. Link That is a pretty strong indicator that homosexuality is genetic as animals are instinctual, IE not capable of making choices.
Edit 2: Oh as for the "realm of Abnormal Psychology"....black people were once thought of as animals at one point in history also...it took strong lobbying to get them elevated to human also.