Greater sexual freedom leads to ... more discipline?

Free markets and free people.  When you let people choose for themselves, "shockingly" they tend to do the right thing. From Slate.com (via Justin Wolfers):

Monogamy rates are probably rising, hard as it may seem to believe, because of sexual liberation. People are cheating less because people are less desperate and unsatisfied. Nowadays you're expected and even encouraged to delay marriage and childbirth and spend your youth experimenting both sexually and in relationships, and so now people who make commitments have both gotten some of the curiosity out of their systems, and they have a better idea of what will make them happy when they do settle down. 

There were simply more bad marriages in the past, created because of the pressure to marry young, and people in bad marriages are more likely to cheat. I also think it's because people are more open about sex. If you have a need that's going unfulfilled, there's more cultural space to deal with it first by opening your mouth and speaking to your partner instead of leaving the house, casting around for someone who can fulfill it. I'd also add that there's less stigma attached to divorce now, so people are far more likely to end a bad marriage in the early stages of it going sour. No need now to go through the process of laying waste to your marriage through cheating and fighting in order to justify the divorce, not when you can simply say, "I'm not happy anymore," and divorce amicably.