W hen I came out with a book making the case for same-sex marriage a few years ago, I expected to spend time selling gay marriage to straight people and marriage to gay people. The surprise was how much time I spent selling marriage to straight people.

By marriage, I mean not just a commitment that two people make to each other. Marriage is a commitment that the two spouses also make to their community. They promise to look after each other and their children so society won’t have to; in exchange, society deems them a family and provides an assortment of privileges, obligations, and caregiving tools ...