Abraham Lincoln once observed that America was founded on a proposition, and that Thomas Jefferson wrote it. He was referring, of course, to the section of the Declaration of Independence that begins, "We hold these truths to be self-evident . . . " The reality, though, is that we are founded on a debate over what Jefferson's proposition means. And the current struggle over gay marriage is but the most recent chapter in that longstanding American argument. The words that started the current controversy were written by John Adams. In 1779, Adams almost single-handedly drafted the Massachusetts Constitution. It was...