Bingo, AustinBill!
"Marriage" is part of Natural Law --- whether you're a Methodist, a Mormon, a Muslim, or a reasonable Multi-culti --- because it stabilizes relations of the procreative kind, and thus unites children securely and publicly with their natural father and mother.
That's where a child gets his identity --- his sense of natural origin --- his place in an organic kinship system --- and the nurturance and providence of a man and woman who will show him how to be at home in this world of men and women.
Marriage is unique. It is not just a set-up between consenting adults (that's what private contracts are for.) It is organically intergenerational. And it is irreplaceable.
I've posted this before but Sam Schulman's seminal article on kinship is a must-read in understanding why marriage is the foundational building block of all societies throughout history and why those who deny it's unique inter-generational and kinship role totally miss the point. Only a society so narcissistic that it views children as a "lifestyle accessory" rather than a sacred gift and life vocation could confuse the inherently sterile with the fertile source of all civilization.