Virtually all American couples, married or cohabiting, expect sexual exclusivity of one another. This article asks why some people are sexually exclusive while others have sex with someone besides their mate. Previous research has linked personal values, sexual opportunities, and quality of the marital relationship to extramartial sex. This paper integrates these findings in a multivariate model that incorporates factors informing sexual decision making as well as demographic “risk factors.” Nationally representative survey data show higher likelihood of sexual infidelity among those with stronger sexual interests, more permissive sexual values, lower subjective satisfaction with their union, weaker network ties to partner, and greater sexual opportunities. With these factors controlled, gender differences are substantially reduced or eliminated, although racial effects persist.