Two things men believe; first, women like them more when they are taken and second, a woman's relationship status influences her interest in the opposite sex. 

At least that second part appears to be true.   

In a new study, women both with and without sexual partners showed little difference in their subjective ratings of photos of men when considering such measures as masculinity and attractiveness.   However, the women who did not have sexual partners spent more time evaluating photos of men, demonstrating a greater interest in the photos. No such difference was found between men who had sexual partners and those who did not.

"These findings may reflect sex differences in reproductive strategies that may act early in the cognitive processing of potential partners and contribute to sex differences in sexual attraction and behavior," said Indiana University neuroscientist Heather Rupp, assistant scientist at The Kinsey Institute for Research in Sex, Gender and Reproduction.

For the study, 59 men and 56 women rated 510 photos of opposite-sex faces for realism, masculinity/femininity, attractiveness, or affect. Participants were instructed to give their "gut" reaction and to rate the pictures as quickly as possible. The men and women ranged in age from 17 to 26, were heterosexual, from a variety of ethnic backgrounds and were not using hormonal contraception. Of the women, 21 reported they had a current sexual partner; 25 of the men reported having a sexual partner. 

This is the first study to report whether having a current sexual partner influences interest in the opposite sex. Other studies have demonstrated that hormones, relationship goals and social context influence such interest.

"That there were no detectable effects of sexual partner status on women's subjective ratings of male faces, but there were on response times, which emphasizes the subtlety of this effect and introduces the possibility that sexual partner status impacts women's cognitive processing of novel male faces but not necessarily their conscious subjective appraisal," the authors wrote in the journal article. 

The researchers also note that influence of partner status in women could reflect that women, on average, are relatively committed in their romantic relationships, "which possibly suppresses their attention to and appraisal of alternative partners."

The study was published in the March issue of Human Nature.  Co-authors include Giliah R. Librach, Kinsey Institute; Nick C. Feipel, Kinsey Institute; Ellen D. Ketterson, Department of Biology at IU; Dale R. Sengelaub, Department of Psychological and Brain Sciences at IU; Julia Heiman, Kinsey Institute.