It is irrelevant to the argument on whether or not it should be "banned"... (remember we are talking, as the title implies, of the entirety of homosexuality, not just marriage).
If all people were cliff jumpers it would be detrimental to the survival of the species, but that is not reason enough to make it illegal, or take away the right to marry of those who do practice this "lifestyle" of cliff jumping. There are any number of things that some people do that if everybody did it would be detrimental to the survival of the species, yet nobody has argued to make those illegal. (Being a Catholic Priest comes to mind, or a Buddhist Monk, a bull rider...). Even some things that are sinful.
Only the one you find icky (sinful) is the one that you want to make illegal.