1. Women use other women's interest in you as a metric for her own attraction. Social proof.
  2. Women are more social than men, the downside of this however is that they're more herd-minded as well. If one of them doesn't find you attractive, good chances are others wont either.
  3. This way you'll aquire the status of "one of the girls" once cemented women in your social circle will always treat you as such and wont even introduce you to their acquaintances.

Ironically, a guy with whom one of these women sleeps with in a one night stand becomes far more intriguing to the rest of them while the man who sits at a book club discussing Margaret Atwood novels with isn't even on the radar. They're relaxed around him because they assume he knows his place and wont try anything.