Five years ago I wrote a post on the homosexual marriage issue in which I tried to explain that it is only within heterosexual relationships that limiting marriage to two people makes sense. That's because a man and a woman can be thought of as two distinct fitting or interlocking parts that combine to form a fruitful (a fertile) union. But once you think that you can have a marriage between two men or two women then there seems to be little reason to believe that you can't have a marriage between three men or three women.
The warning here is that if a society embraces homosexual marriage, then it is on a slippery slope to marriage between more than two people.
It's not easy to predict how long such effects might take. But already we have news from Colombia that a marriage between three men has been given legal recognition: