What is "Marriage"?

I don't think many dictionaries, even a few decades ago would have recognized definition 2.b.:

mar·riage [mar-ij] noun

1. a legally, religiously, or socially sanctioned union of persons who commit to one another, forming a familial and economic bond: Anthropologists say that some type of marriage has been found in every society, past and present.

2. a. the social institution under which a man and woman establish their decision to live as husband and wife by legal commitments, religious ceremonies, etc.

b. a similar institution involving partners of the same gender, as in gay marriage; same-sex marriage

So is marriage open to redefinition, is it purely human construct?
If the sex of the participants can change with loosening of societal restrictions, and to prevent it is a violation of rights, of freedom, of our notions of fairness -- why not the number?

Why must marriage be two joined as one?

How dare we legislate against marriages of three or four persons?
Why can't I be married to Himself and to someone else? how about my brother? is there any logical reason the law should not allow me to marry him?

It could make child care easier.

Perhaps adoption agencies should be forced to place children with consortiums , (consortia?) of more than two people.

Your bridge group could adopt together. Heather Has Five Mommies.

Why not? is there a logical consistency to what is acceptable, legally, morally, socially, and what is not at this moment in history?