Their findings echo the plot of 1989 film When Harry Met Sally, in which Harry, played by Billy Crystal, tells Meg Ryan's character Sally: "Men and women can't be friends because the sex part always gets in the way."
Scientists said the results of the study showed "potential negative consequences" for people in long-term relationships.
But a deeper-lying mating instinct, developed over hundreds of thousands of years of evolution, could make them more complicated than they might seem.
In the study, 88 pairs of young male and female friends were asked to rate their attraction to each other on a secret questionnaire.
The men also assumed that the women were more sexually interested in them than they actually were - and the women tended not to realise this.
Researchers found that single and attached women had the same level of attraction to their male friends - but attached women only wanted something more if their own relationship was on the rocks. Women were also less attracted to attached men, the study found.
The authors of the research, from the University of Wisconsin, said films and television programmes had helped instill the idea that normal friends could become "friends with benefits" - that is, who have sex with each other.
What Is Sexual Orientation?Sexual orientation is an enduring emotional, romantic, sexual, or affectional attraction toward others. It is easily distinguished from other components of sexuality including biological sex, gender identity (the psychological sense of being male or female), and the social gender role (adherence to cultural norms for feminine and masculine behavior).
Sexual orientation exists along a continuum that ranges from exclusive heterosexuality to exclusive homosexuality and includes various forms of bisexuality. Bisexual persons can experience sexual, emotional, and affectional attraction to both their own sex and the opposite sex. Persons with a homosexual orientation are sometimes referred to as gay (both men and women) or as lesbian (women only).
Sexual orientation is different from sexual behavior because it refers to feelings and self-concept. Individuals may or may not express their sexual orientation in their behaviors.