TORONTO—Gay or straight, male or female—everyone is having fewer affairs now than they were in the 1970s. According to a new study presented here today at the annual convention of the American Psychological Association, extramarital (and extra-partnership) sex is way down, and discussion about the topic within couples is way up.

Psychologists at Alliant International University in San Francisco and their colleagues compared survey responses from two large groups of couples, self-categorized as gay men, heterosexual men, lesbians or heterosexual women. About 12,000 people answered the relevant questions in 1975; close to 1,000 participated in 2000. The average length of the relationship at the time of the survey varied between groups, from about four and a half years for lesbians, almost seven years for gay men and about 14 years for heterosexual couples in 1974 to nearly 11 years for lesbians, 13.5 years for gay men and almost 20 years together for straight couples in the 2000 survey.

The overall result was unambiguous—monogamy rates have skyrocketed. But the groups still show dramatic differences in how often they cheat or have sex outside of their primary relationship. Around 82 percent of gay men reported extra-partnership sex in 1975, whereas 59 percent did in 2000—a significant decrease, but still that later rate is more than four times higher than comparable rates found among straight men (14.7), straight women (13.5) and lesbians (8.2). Those groups’ rates are down from percentages in the mid-twenties in 1975.

It's worth pointing out, however, that 43.7 percent of those gay men said they "discussed sex outside the relationship and decided that under some circumstances it is all right." Only 5 percent of lesbians and about 3.5 percent of straight couples had a similar agreement. Again, all groups report many fewer of these open relationships than they did in 1975, when about 20 percent of straight couples, 34 percent of lesbians and nearly 68 percent of gay men agreed to forgo monogamy.

And the percentage of couples who are decidedly closed to sex outside the relationship—they discussed extra-partnership sex and decided that "under no circumstances is it alright"—just about doubled in every group (from around 43 percent in 1975 to around 80 percent in 2000) except in gay men, among whom it more than tripled (13 to 44 percent). "It was surprising to us that in all groups, the trend is toward monogamy," said Gabrielle Gotta, lead author of the study.

So what’s the bottom line? There is certainly a large movement toward monogamy—most likely due to awareness about HIV and other STDs, speculate the authors—but every couple is different. More couples are discussing their options now than they were in 1975, and figuring out what works for them.

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