"I don't think giving unmarried couples legal recognition would destroy the institution of marriage... straight couples who don't want to marry for whatever reason"

I don't understand this. If the couple has legal recognition, in what way is that not marriage as we currently understand it? No is required now to have a religious ceremony.

jpk, are you advocating "if in the course of ordinary biological acts, they can beget offspring, then let them marry?" & if not, no marriage? That's kind of hard on post-menopausal women.