But seriously, though most people think that polygamy and group marriage are the next thing down the slippery slope, I’m not so sure. If you redefine marriage as a contract between any two people, that still rules out marrying five wives or your horse. What it leaves open is incest, and I think incest is next up on the agenda. Laws about the degree of consanguinity considered incestuous already vary from state to state, and in the homosexual case (in which there’s no prospect of natural offspring), what possible basis could you have for forbidding a man to marry his brother, son, nephew, or male first cousin?