The most destructive form of romantic betrayal is sexual infidelity. A partner who cheats on you has broken a fundamental sacrament—the spoken or unspoken pledge to remain faithful. Not surprisingly, many people react to infidelity with a strong urge to leave their partner. One study found that more than 20 percent of American women and men would consider divorce if a spouse passionately kissed someone else, more than 30 percent would consider divorce if their spouse had a romantic date with another person, and more than 60 percent would consider divorce if their spouse had a serious (sexual) affair (Shackelford & Buss, 1997). Whether or not a sexual dalliance is planned matters little: cheaters’ original intentions have no impact on subsequent feelings of blame by their partners (Mongeau, Hale, & Alles, 1994).

Although both men and women view infidelity as treasonous, their perceptions diverge when they’re asked to compare sexual with emotional cheating. Infidelity researcher David Buss presented study respondents with the following dilemma (Buss, Larsen, Westen, & Semmelroth, 1992). Imagine you discover that your partner has become interested in someone else. What would distress you more: your partner forming a deep emotional attachment to that person, or your partner enjoying passionate sex with that person? Sixty percent of men said that sex would upset them more, but 83 percent of women said they’d find the emotional attachment more distressing. The same pattern of results was found in samples of men and women from Sweden, the Netherlands, Germany, Korea, and Japan (Buss et al., 1999; Buunk, Angleitner, Oubaid, & Buss, 1996; Wiederman & Kendall, 1999).