http://dx.doi.org/10.1037/ebs0000165

In a sample of college couples, we examined the frequency of extrapair copulations, how these differ from intercourse with committed partners, and how knowledge of the other person’s relationship status affects sexual behavior. More than 25% of both men and women reported engaging in 1 or more extrapair copulations. Those who cheated reported greater arousal, but the duration of intercourse was not affected. Both sexes achieved greater sexual satisfaction from extrapair copulations. When men engaged in intercourse with women they knew were in a committed relationship, thrusting was quicker, deeper, and more vigorous. Men also reported more intense orgasms and attempted to prolong intercourse for as long as possible when having sex with someone who was in another relationship. Women did not. Differences in various parameters of extrapair orgasmic experiences (latency to orgasm, frequency of orgasm, intensity of orgasm, and orgasm duration) were consistent with a priori predictions based on sex differences in fitness maximization