It's a continuum. The question is, where is the end? Remember the German cannibal couple? At least the eaten one was involved "consensually". /sarcasm.
Polyamory -- say, two men and three women in a "marriage." Legalized pedophilia and incest, as well. Eventually, if marriage means everything marriage means nothing -- people won't even bother getting married. In some European countries, shack up couples have almost as many rights as married ones.