Yglesias:

“Don’t make out with another dude” is really easy for most dudes. It’s very hard for some, of course, but most people aren’t gay and thus have absolutely no difficulty adhering to moral injunctions against gayness. A political movement that ran around talking about the sanctity of marriage and how divorce should be illegal and divorced people should be disgraced and run out of public life would have trouble catching fire. But almost everyone can stand up for “traditional morality” on the cheap by heaping scorn on a relatively small minority.