You could say the same exact thing about gay marriage ten years ago. Forty years ago it was thought of as abhorrent like pedophilia. If you would have asked anyone back then if it would be legal one day they would have laughed I am sure.
There are people now pushing for the age of consent to be lowered. There is an attempt to normalize pedophilia on the fringes. Forty years from now it will be legal.
Why can't a man marry his mother or sister? It would cause issues genetically speaking but reproduction is not the purpose of marriage now. Societal norms change. Just wait and see.
I don't have a problem with gay marriage. I just see that it kicks in the door for everything else.