Biologically there is almost no reason for a man not to have sex with a woman. The costs on his side are close to zero, apart from the risk of catching disease. For a woman there is a much higher biological cost, since pregnancy puts an extreme strain on her body, comes with the risk to die in child birth etc. So usually women will be more reluctant to accept a sexual partner unless he would also make a suitable mate.  That changes when it comes to relationships - here the situation is a lot more balanced for men and women. The cost for men to raise children is about the same as for women, everybody has only one lifetime to spend. So they apply similarly high standards, wanting a fit, attractive mate - someone who is able to contribute equally to the family.  She is applying female expectations to men, thinking that finding someone who is willing to have sex with her must then mean that they find her attractive too, and therefore there should be a potential for a relationships.