A functional account of marriage vows

What does a couple have to validly promise each other, explicitly or implicitly, in order for those promises, when appropriately ratified by authority, to give rise to a marriage?

This is a hard question as to the specifics. But we can at least give a start of a functional characterization:

• Marriage vows are that complex of binding commitments that in fact makes it prima facie permissible for a couple to engage in intercourse.

One might want to add that the complex is natural, either in the David Lewis sense (in which case it is opposed to gerrymandered) or in the natural law sense (in which case it is that which is consonant with our nature).

Of course, this account can only have plausibility if uncommitted sex is wrong. I think that prior to the 20th century in the West, this functional characterization would have been seen as quite plausible, and I am still inclined to think it is correct.

A functional characterization is not, of course, a definition. Thus someone who disagrees with this characterization can still be talking about the very same thing when using the words "marriage vows" as someone who accepts this characterization.