In some cultures, people tend to believe that it is very important to be sexually exclusive in romantic relationships and idealise monogamous romantic relationships; but there is a tension in this ideal. Sex is generally considered to have value, and usually when we love someone we want to increase the amount of value in their lives, not restrict it without good reason. There is thus a call, not yet adequately responded to by philosophers, for greater clarity in the reasons §why (...) it might be reasonable for a couple to adopt a policy of sexual exclusivity. This article argues that we cannot justify the demand for sexual exclusivity by a need to protect the relationship from the risk of one partner ‘trading-up’, or by appealing to jealousy. However, sexual exclusivity can be intelligible if it supports the romantic relationship and helps to distinguish it from other relationships. Nonetheless, sexual exclusivity ought not to be the hegemonic social norm that it currently is in some societies because this diminishes the potential value it might have and gives the idea of faithfulness the wrong focus. (shrink)