"What's so magical about two?"

Here at opinion over at WJS Dana's Mack "What now for marriage"there is a comment that pretty much sums up that we're not adding to marriage by rather making it pointless.

Marriage, for thousands of years, has been - at its best - a spiritual covenant between a man and a woman. At its worst (sadly, too often) marriage has been an abomination of broken relationships (and abusive exertions of power) between multiple men and women (polygamy and/or serial marriages). To radically redefine marriage as now being between a man and a man or a woman and a woman is simply to embrace the decay of the ideal. How tragic. Yet not surprising. Why not now continue the redefinition? And then, why have marriage mean anything at all? What possible reason would there be to keep a man from marrying his sister? Or mother? Or father? Or a woman from marrying her son? Or daughter? Seriously, why not? Toss aside the "reproductive dangers" argument due to contraception (or older partners). Why should there by any limits at all to marriage if it is to be redefined?! And as long as we're opening things up, why should marriage be, any longer, between TWO people? What's magical about "two?" Why shouldn't a person be allowed to marry two or three other people? Or everyone on their block? Really. What's the point of marriage at all if we're going to redefine it, according to whatever standards come down the pike??!