Heck; all it would take is another Roe v. Wade, only this time instead of the USSC handing out the edict that a State cannot prohibit abortion (by viewing it as murder) that the States cannot restrict the natural rights of people to marry the one(s) they love
(or some similar BS).
The question is this...if you opened the door to threesome marriages or polygamy....how many folks would show up in the next month to get a license and married in such a fashion? I think this is the curious part of the approaching evolution.
In Utah....maybe a hundred such marriages would occur in the first month...but what about states like Florida?