Both "having sex" and "having a relationship" involve emotional groundwork. The latter tends to diminish the effort for the former. If you're jettisoning your "relationships" regularly you're putting in an awful lot of "needless" effort from an economic point of view. It's an irrational choice that only makes sense if there is a unique draw to the initial groundwork of having sex or a unique repulsion to the repeated groundwork of having a relationship. As flirtation doesn't necessarily have to result in copulation for the endorphins to flow, the latter is a more likely explanation.The reality is that I have personally never met or heard of anyone hyper promiscuous (in terms of endless one night stands with strangers, relationships never lasting more than weeks, large amounts of casual partners) who didn't have some kind of issue.