Is 2 a Magic Number?

Why does a "marriage" consist of two people, as a rule?
Not talking about "reality" television, (or Mitt Romney's grandpappy,) or King Solomon and his, what? 700 wives?
Or is it a thousand , but seven hundred of them were royals?

Either way....

No, I'm talking about regular folks, I might even have said normal folks, if the word "normal" weren't hate speech in the context of.... oh, everything having to do with humanity.

Even Henry Two-door, (the second was for sneaking those young ladies in pre-connubial state out when the current queen came a-callin',) the Supreme Head of the Church of MakingItUpAsWeGoAlongGivingMoreWeightToWhatTheLittleHeadWantsThanWhatTheBigHeadKnowsIsRight Church of England never presumed to have two wives at the same time, and he was pretty darn self-indulgent.

"When the King does it, it's NOT fornication..."

And I guess I should only be talking about the Western world, there're plenty of other societies that celebrate, or at least allow polygamy.

So, given those parameters - why is "two" the magic number?

If the complementarity of the two sexes is not inherent in the framework set for prospective spouses, I can see no reason for limiting marriage to sets of two, division of labor certainly doesn't demand it, and legally child rearing is looked on as a task for one, nowadays.

So why not a task for committee?
Why not allow a group of five to adopt? (Make birth certificates a little larger, lines for a few more names...)

Why shouldn't a "marriage" consist of... well, Jules and Jim and Catherine?

No reason I can think of.

If to be considered a "marriage" a relationship need not be between a man and a woman, why should it have to be two people at all?