Folks of all sexual orientations who are in committed relationships have become more monogamous over time, or that's what a study that was published Family Process found. There are some hinky things in the reporting on this piece at USA Today. For instance, the only heterosexual couples mentioned are married, but gay couples who are committed but have no formal union were also recorded. Additionally, the reporting conflates cheating with sex outside of the relationship, even though many couples have an understanding that allows for outside relationships. In fact, nonmonogamous cultural norms in gay male culture go a long way toward explaining why they're far more likely to have sex outside of a committed relationship than everyone else.
Still, even with those caveats in place, the results of this survey are stunning. The rate of sex outside of the marriage has dropped for every category of people studied dramatically between 1975 and 2000. Twenty-eight percent of straight men in 1975 had sex with a woman outside of their marriage, but in 2000, it was only 10 percent. For straight, married women, the rate dropped from 23 percent to 14 percent. For gay men, 83 percent to 59 percent, and for lesbians, 28 percent to 8 percent. The USA Today article focuses mainly on gay couples and how the mainstream acceptance of homosexuality has a lot to do with increasing rates of monogamy. There's a lot to think about there, since it is true that cultural acceptance has introduced far more stability into the lives of gay people, and the gay marriage movement has also increased the pressure to value monogamy.
But that doesn't account for all of the change, since straight people have grown far more monogamous, too. The explanation for this offered in the USA Today article doesn't ring true to me, that it's all about increased awareness of STDs. They had STDs in 1975, and people worried about them then, too. Plus, the unintended pregnancy rate was much higher then, and most research I've found suggests that straight people worry far more about pregnancy than any STD. AIDS really doesn't account for the difference, since most straight people really don't see themselves at risk, even if they're cheating. They worry more about the stuff they worried about in 1975: the clap, herpes, warts.
No, I think something else is going on here. 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.