If you honestly believe that gay marriage will result in anything other than two men or two women getting married than you are beyond hope.
It's not a question of gay marriage leading to polygamy, etc. It's a question of whether a particular method to legalize it will create a legal precedent that can be used by adherents of those other forms of marriage to legalize those too.
A statute or a referendum legalizing gay marriage would be unlikely to lead to such precedent. A badly worded court case (finding, for example, that marriage is a "right") could.
For example, going to back at least to
2006, polygamist rights groups have been plotting to use court cases that create gay rights to advance the legalization of polygamy"
The U.S. Supreme Court's 2003 decision in Lawrence v. Texas, which voided laws criminalizing sodomy, also aided polygamy's cause because it implied that the court disapproved of laws that reach into the bedroom.
Since then, liberal legal scholars, generally no friend of the polygamists' conservative-leaning politics, have championed decriminalization. One of them is Jonathan Turley, a law professor at George Washington University who has written two op-eds for USA Today calling for the legalization of bigamy -- and same-sex marriage.
If it's already being worked on, it's hard to say it's "never" going to happen.
