For those who want evidence that procreation is an important public purpose for marriage, A quick sampling:

From U.S. law.

"[T]he first purpose of matrimony, by the laws of nature and society, is procreation." Baker v. Baker, 13 Cal. 87, 103 (1859). "he procreation of children under the shield and sanction of the law" is one of the "two principal ends of marriage." Sharon v. Sharon, 75 Cal. 1 (1888) (quoting Stewart on Marriage and Divorce, sec. 103. "Procreation, if not the sole, is at least an important, reason for the existence of the marriage relation." Davis v. Davis, 106 A. 644, 645 (N.J. Ch. Div. 1919). "The great end of matrimony is . . . the procreation of a progeny having a legal title to maintenance by the father." Laudo v. Laudo, 197 N.Y.S. 396, 397 (App. Div. 1919); Poe v. Gerstein, 517 F.2d 787, 796 (5th Cir. 1975) ("[P]rocreation of offspring could be considered one of the major purposes of marriage. . . ."); Singer v. Hara, 522 P.2d 1187, 1195 (Wash. App. 1974) ("[M]arriage exists as a protected legal institution primarily because of societal values associated with the propagation of the human race."); Baker v. Nelson, 191 N.W.2d 185, 186 (Minn. 1971), appeal dismissed for want of a substantial federal question, 409 U.S. 810 (1972) ("The institution of marriage as a union of man and woman, uniquely involving the procreation and rearing of children within a family, is as old as the book of Genesis."); Heup v. Heup, 172 N.W.2d 334, 336 (Wis. 1969) ("Having children is a primary purpose of marriage."); Zoglio v. Zoglio, 157 A.2d 627, 628 (D.C. App. 1960) ("One of the primary purposes of matrimony is procreation."); Frost v. Frost, 181 N.Y.S.2d 562, 563 (Supr. Ct. New York Co. 1958) (discussing "one of the primary purposes of marriage, to wit, the procreation of the human species."); Ramon v. Ramon, 34 N.Y.S. 2d 100, 108 (Fam. Ct. Div. Richmond Co. 1942) ("The procreation of off-spring under the natural law being the object of marriage, its permanency is the foundation of the social order."); Stegienko v. Stegienko, 295 N.W. 252, 254 (Mich. 1940) (stating that "procreation of children is one of the important ends of matrimony"); Gard v. Gard, 169 N.W. 908, 912 (Mich. 1918) ("It has been said in many of the cases cited that one of the great purposes of marriage is procreation."); Lyon v. Barney, 132 Ill. App. 45, 50 (1907) ("[T]he procreating of the human species is regarded, at least theoretically, as the primary purpose of marriage . . ."); Grover v. Zook, 87 P.638, 639 (Wash. 1906) ("One of the most important functions of wedlock is the procreation of children."); Adams v. Howerton, 486 F. Supp. 1119, 1124 (C.D. Cal. 1980), aff'd 673 F.2d 1036 (9th Cir. 1982) (observing that a "state has a compelling interest in encouraging and fostering procreation of the race");

A New Jersey court waxed lyrical on this point: "Lord Penzance has observed that the procreation of children is one of the ends of marriage. I do not hesitate to say that it is the most important object of matrimony, for without it the human race itself would perish from the earth." Turney v. Avery, 113 A. 710, 710 (N.J. Ch. 1921)

Some evidence on the anthropological point: "Although the details of getting married -- who chooses the mates, what are the ceremonies and exchanges, how old are the parties -- vary from group to group, the principle of marriage is everywhere embodied in practice. . . . The unique trait of what is commonly called marriage is social recognition and approval . . . of a couple's engaging in sexual intercourse and bearing and rearing offspring." Kingsley Davis (ed.), Contemporary Marriage: Comparative Perspectives on a Changing Institution (New York: Russell Sage Foundation) (1985).

"Marriage is a universal social institution, albeit with myriad variations in social and cultural details. A review of the cross-cultural diversity in marital arrangements reveals certain common themes: some degree of mutual obligation between husband and wife, a right of sexual access (often but not necessarily exclusive), an expectation that the relationships will persist (although not necessarily for a lifetime), some cooperative investment in offspring, and some sort of recognition of the status of the couple's children. The marital alliance is fundamentally a reproductive alliance." Margo Wilson & Martin Daly, Marital Cooperation and Conflict, in Evolutionary Psychology, Public Policy and Personal Decisions 197, 203 (Charles Crawford & Catherine Salmon eds., Lawrence Erlbaum Assoc. 2004)

Its certainly possible, guys, to argue that marriage's public purpose has changed, or is no longer relevant, or that same-sex marriage doesn't threaten this interest.

But really not to say that I'm just making up this whole procreation and marriage thing.