Already banned per LDS
And the logic of the argument is absolutely sound, as Conservatives have indeed recognised for a long time. If there can be no moral objection to same-sex "marriage"; if our laws are just the products of bias and bigotry (as is alleged); if practical objections (disease etc) can be overcome or carry no weight, then there can be no objection to it taking place. And by implication, if same-sex "marriage" is valid, then why not polygamy? If the institution of marriage is made more "elastic" by removing the requirement for different genders, then what objection can be made to polygamy? The people involved are consenting adults, they enter into the estate of their own free will - without the moral underpinning there can be no objection.
The next stage will be necrophilia perhaps, or bestiality. Maybe eventually paedophilia, possibly brought in by lowering the age of consent. There are already fears that children are "growing up too quickly" and becoming sexualised at ever earlier ages - condoms being handed out at age eleven and so on. By the end of this century (assuming the US is still around) people will probably have the "right" to "marry" their own wallpaper.