A few years ago, I had a friendly argument with a colleague who believes same-sex marriage ought to be legalized. I made the standard argument that if our current marriage laws — this was before gay marriage had spread to some states — were based on nothing but irrational prejudice, as he maintained, then there was no reason to deny some sort of marital union to polygamists and polyamorists. Ridiculous, he said.: “Marriage is between two people.”

“But why?” I pressed. “Isn’t that arbitrary?”

He honestly didn’t understand my objection. He thought it inconceivable that any court would grant what to him was sheer crackpottery. The very idea of polygamist marriages being sanctioned in law! Inconceivable! Which is what they said not 20 years ago about same-sex marriage. Anyway, I’ve found over the years that people who argue that restricting marriage to opposite-sex couples is wrong because bigoted and because mutual consent should be the only barrier to marriage often do not know what to say when the slippery slope to plural marriage is mentioned. They don’t seem to appreciate how the premisses and the logic that leads them to back same-sex marriage — particularly the belief that marriage is merely a social institution — makes it difficult to make a principled argument against other alternative marital arrangements.

This conversation came to mind just now reading, via First Things, Matthew J. Franck’s piece emerging out of a public debate he had with a same-sex marriage proponent. Excerpt:

She had begun, in her prepared remarks, by calling on a standard of “rights” that cannot be defeated by appeals to “tradition.” And she had mocked judges who, in the early decisions on the case for same-sex marriage, had simply turned to a dictionary definition of marriage.

Yet, in her response to my point about plural marriages, Gorenberg herself turned immediately to tradition and to received definitions. Marriage just is a “binary institution,” she asserted, and changing that fact would entail all sorts of inconveniences. (The historic existence of polygamy in many places is proof that these inconveniences are not insurmountable, but this did not slow her down.)

Why mere tradition was now owed such automatic allegiance, she did not pause to explain. Now the prospect of altering a “whole raft of laws” associated with marriage filled her with horror and incredulity. She seemed quite oblivious of the fact that she was making my argument for me. Where was her concern about changing all the details and complexities of a forest of family law planted thick with assumptions about husbands and wives, mothers and fathers, always of opposite sexes?

In her nimble way, having shed the drag-chute of principled consistency, Hayley Gorenberg demonstrated the great strength of the movement for same-sex marriage. She did not have anything to offer in answer to the questions “What is marriage? What is it for? What are its boundaries? Whom should it include?” She does not need answers to such questions. All she needs is an argument for the moment, for the cause, for the victory she wants right now. If her argument is all sound and fury, signifying nothing, that is not her concern. The cause is self-justifying.

I well understand why people of good will would want to extend marriage to same-sex couples. Many of us know gay couples, like them, don’t see why they should be denied what straight married couples enjoy. I get that. Yet I fear many of these advocates don’t understand what they concede — what they must concede — to achieve that.

Why, for example, should a brother and sister who have agreed to undergo sterilization as a condition of their marital union be denied the right to marry, if that is their wish? There would be no chance of genetically damaged issue. So why not?