This article reports the results of two studies looking for evidence of social norms governing the behavior of college-age heterosexual men and women toward their friend's romantic partners. Respondents read scenarios describing a protagonist trying to start a relationship with a friend's romantic partner and were asked to rate their approval or disapproval of that behavior. The studies varied information about the sex of the protagonist, friend and romantic partner, and the relationship of the romantic partners. Controlling for respondents' race and social class, we found that for both men and women a social norm exists only when a third party was viewed as interfering with a couple's relationship, which was described as clearly known and shared by all parties. The lack of a norm when there was ambiguity about the level of commitment between romantic partners is interpreted as the result of conflict among three alternative ideologies of loving relationships.