The perverts realize they cannot get every perversion legalized rapidly. Most sensible people are still put off by things like bestiality, pedophilia, and even polygamy. So the perverts are going at it a small step at a time, despite statements to the contrary, claiming that sodomite "marriage" is not a stepping-stone to other absurdities.
They have started with homosexual "marriage". The next step is a hybrid of straight marriage mixed with queer marriage called a "triad". Hey, if straight marriage is okay, and homosexual marriage is legal, who can argue about a combination of the two? This will move the bar so they can try for the next perversion.
Queer marriage is indeed a slippery-slope to other sexual insanity. We've posted dozens of items proving that claim.