Many of you are missing the point that stirs is trying to make.
He is not saying that this law will entice or lead to people wanting to commit incest, polygamy, etc. He is saying that a law written so vaguely (basically defining a marriage partner as anyone of your chosing) could provide a legal means and support for those who do want to participate in incest, polygamy, wedding underage kids, etc. They could and will use this law as a means to legally do it....until the law is eventually rewritten.
What he's doing is playing the "We're not protecting slavery, we're protecting States Rights" type shell game.