Thanks by the way for all your comments, especially this one: "Before belly-flopping into an already acrimonious debate on same-sex marriage, would someone please define exactly what marriage is supposed to accomplish. Perhaps then the debate can proceed on firmer terra."
Here’s my short answer: marriage serves many private and individual purposes. But its great public purpose, the thing that justifies its existence as a unique legal status, is protecting children and society by creating sexual unions in which children are (practically) guaranteed the love and care of their own mother and father.
The vast majority of children born to married couples begin life with their own mother and fathers committed to jointly caring for them. Only a minority of children in other sexual unions (and none in same-sex unions) get this benefit.
Sex makes babies. Society needs babies. Babies need fathers as well as mothers. That's the heart of marriage as a universal human institution.
Please note: Procreation is not the definition of marriage. It is the reason for marriage’s existence as a public (and yes legal) institution. People who don’t have children can still really be married (just as people who aren't married can and do have babies).
But if sex between men and women did not make babies, then marriage would not be a universal human institution, or a legal status in America. Yes, many people like intimacy—is that a good reason for the government to stamp the good housekeeping seal of approval on certain intimate relationships, but not others?
(Note: I’m aware the short description above does not answer all legal arguments about equal protection. Patience. BTW, the legal debate would be sharpened if more people participating in it distinguished between the individual interest and the state interests in marriage).
For a longer explication of this argument see my debate with Andrew Koppelman in the U. of St. Thomas Law Review, "(How)Will Same-Sex Marriage Weaken Marriage as an Institution?"