Heterosexual marriage has had a tough few decades, but its one success story is the declining divorce rate among the upper middle class. This decline, tellingly, has gone hand in hand with steadily rising disapproval of adultery.

There’s a lesson here. Institutions tend to be strongest when they make significant moral demands, and weaker when they pre-emptively accommodate themselves to human nature.

Critics of gay marriage see this as one of the great dangers in severing the link between marriage and the two realities — gender difference and procreation — that it originally evolved to address. A successful marital culture depends not only on a general ideal of love and commitment, but on specific promises, exclusions and taboos. And the less specific and more inclusive an institution becomes, the more likely people are to approach it casually, if they enter it at all.