Well, as I see it, the whole issue of SSM is based upon claims of Constitutional rights granting "Protections", although which specific rights, under the Constitution, weren't made exactly clear in the document. Vague cross-references to the 14th Amendment notwithstanding. But the inference is that gays aren't allowed to make selections, based upon violations of these rights. They claim that they should possess the rights to choose who they wish. But, the Mormons are also granted these same vague rights, except their religious rights are being violated by the same governmental overstepping. And the Mormons have actually shown that they consider this a violation of "Religious Freedom", more specifically granted under the C.<quoted text>
I just don't how a group could come before a court and argue a case that would support incestuous marriage. With all that is known about possible medical issues of offspring, I just don't see it happening.
Bigamists may at some point try to bring their case before a court, but how can they base their on same-gender marriage? Gays aren't asking for the right marry multiple partners.
The US addressed plural marriages with the 1862 Morrill Anti-Bigamy Act and in the Supreme Court case Murphy v. Ramsey.
There is an attorney in Utah, Jonathan Turley, who has files suit against the state; asking that plural marriages become legal. I don't know how far he's gotten.
Anybody in the U.S. has the right to pursue whatever rights they believe they are due.
I just don't think that same-gender marriage is going to lead to or open the door to incestuous or plural marriage.
As per incestuous marriage, SSM arguments have clearly claimed that procreation is not enough to validate or invalidate marriage. Birth control being available, and medical procedures that can render sterility. Probably a bit of a ramble, as I am no Constitutional scholar, but there aren't too many of those in here, anyway.
As to the 1862 Morrill Anti-Bigamy Act, and Murphy v. Ramsey, I would counter that we used to have Sodomy Laws on the books and that these, also, were repealed. And that the same arguments that had these laws repealed could also serve the same purpose for the P and I crowd, eventually. Views change over time, and if we are to allow for changing to suit one, why not others? Upon what basis does the government stand, to justify meddling with marriage laws, at all?