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.<quoted text>
I honestly can't agree with the "slippery slope" label, as you're right in the rest of your post. I can't see the issue any other way, except to note it as a logical implication, not a slippery slope.
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.