Well speaking purely biologically, living life's purpose is to reproduce. In essence a gay person falls outside this, and resorts to human-made institutions of adoption to "reproduce".
Its most likely a brain thing, where they have the sex drive of the opposite sex, like a gay man having the sex drive of a straight woman.
In the end, nothing wrong with being gay, and its most likely non curable with our current technology if your brain has the "opposite" sex drive.