I couldn't disagree more strongly. "Our established normal human function of mating" is immaterial to nature. Look at our close relations, the Bonobos, and their free for all sexual behavior. Look at all the varied examples of homosexual behavior among other mammals. Look at the history of homosexuality in humankind. 

Conversion therapy works in a very small percentage of people because, as Dr. Robert Epstein's work explains, sexual orientation is on a continuum from 0% to 100%, and some people are only very slightly homosexual in the first place. Someone who 90% homosexual is not going to become heterosexual no matter how much conversion therapy is tried. 

And common sense tells us why the distress - it's all about acceptance. If you are not accepted by your family or society, then of course you are going to feel distressed. So obvious!