Oh that's a good one. How about furthering it with....the government should stay out of the bedroom. It should not restrict or sanction physical relationships between two consenting adults.
Also you could point out the dire statistics of divorce amoung heterosexuals, the costs, the devastation, and the implied consequences to gay married couple who tend to change partners at a higher rate than heteros.
Of course, no such government would last very long as their society passed away within a few generations.
Perhaps that's why there are no "libertarian" nations on this planet?
I agree that this is the best argument. The government "liecensure" of any union is inappropriate.