In the end, does it really matter if it's natural or something done by choice?
Has anyone here been in love? Deeply in love? Love was not explained scientifically (was it?) as far as I know and my guess is that being human, homosexuals fall in love the same way heterosexuals do. Why would we even try to "cure" a girl that is madly in love with another girl, prepared to do everything for her, just as me or you were capable of doing when we were in love with a girl? Is there a cure at all?
Personally I wouldn't like to watch two guys doing it. The solution for me is easy - do not watch guys doing it. If anyone does it in public, the same should be done as with hetero couples - fine them.
Gay couples are no danger for the human race. With sperm banks and everything, even if we all turn gay one day, we will still have men to provide the sperm and women to carry a baby. Just let these people be. They don't present a danger for you, as much as you find the image of them doing it disturbing.