Very Complex

No simple cause for sexual orientation has been conclusively demonstrated, and there is no scientific consensus as to whether the contributing factors are primarily biological or environmental. Many think both play complex roles.[1][2]. The American Academy of Pediatrics and the American Psychological Association have both stated that sexual orientation probably has multiple causes.[3][4] Conclusive proof of a biological cause of sexual orientation would have significant political and cultural implications. [5]

Just put the word "Gay" into a Google Search and it's mind boggling to read all the research that has and is still being done to determine why an individual is Gay.
My feeling is that regardless of why a person is Homosexual has nothing to do with how they should be treated by society. Yes there are some women and men who decide to experiment for the thrill of it and aren't gay this isn't what we are referring to.
My children are grown and are all in Heterosexual relationships. I would not have loved any one of them any less if they had been gay. I know that it's not easy for a parent to accept and the reason is the intolerant smug people that will make life difficult for that child.
My brother who is deceased was gay and it breaks my heart that he was never able to discuss it with my family and therefore had to feel so isolated. There isn't a day that goes by that I don't regret not having sat down and talked about it with him.
Why didn't I ??