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.