Townsend said:
Why shouldn't we accept bigamy? Why does a person's personal life have to be socially acceptable?
Oh, but I didn't have anything a priori against bigamy either. The point is that the civil marriage gives certain rights to a group of (2 ?) people ; it is in fact the only reason to subscribe to a civil marriage.
They are essentially 2-fold:
1) financial aspects, like lower taxes on heritage when one of the partners dies
2) co-parentship when (one of the?) females in the couple gives birth.
Now, there's a long-standing tradition of 1 man and 1 woman as such a basic cell which doesn't even find its origin in religion but finds it in Darwinism: promote the chances of transfer of the genes to the future generation. The optimal team is then of course a man and a woman, because their kin has 50-50 gene content, so this man and this woman will optimize the transfer of their genetic material to the future (will care best for their kin).
All other combinations will do less good. In a situation of one male with several females, the females who didn't give birth to certain children will have of course all advantage to care more about their own children than of the children of the other spouses of their male ; on the other hand, the male would like to see ALL its children cared for (he has 50% of the genetic material in all of them). So this situation is only advantageous if the male is very dominant over the females. In fact, what males hate the most are unfaithful women because then they are caring about kin that has NOT their genetic material - hence the usually very harsh conditions that are put traditionally on adulterous women by a male-dominated society.
See, all tradition (cast into iron by religious traditions) have to do with optimal gene transfer to the next generation, and marriage is one of those traditions, and explains why those traditions have a strong preference for the 1man-1woman situation.
From the moment you leave that reason (such as gay marriages) you can relax in fact all conditions, and have a general kind of contract between members of a group.