Here is a suggestion: put the onus on the other debater to formulate a principle out of the concept you can't legislate morality. You should be able to pick apart any edifice he constructs because it is an empty concept.
Another suggestion: marriage has existed as the union of one man, one woman for thousands of years. The states passed legal statutes about an existing institition. Who has decided that the public should assert their dominance over tradition?
The biblical patriarchs often had multiple wives - two daughters of Laban spring immediately to mind.