The reality is not quite so simplistic but there really is something to the idea that women find a man more attractive if he already has a girlfriend. This is in part social proof that he’s worth pursuing and that he’s interested in a committed relationship and it’s a matter of what Robert Cialdini calls the law of scarcity in that something that is perceived to be of higher value (due to demand) or is rare or limited has a much higher perceived value than something – or someone – that appears to be abundant or not in demand. A single man with no attachments has no social proof and he appears to be a non-challenge in terms of access.
See http://jezebel.com/5847003/fake-girlfriend-helps-guys-pick-up-chicks and http://www.huffingtonpost.com/2011/10/03/fake-girlfriend-texting-service_n_992152.html and more for recent bits on the gambit of dressing one’s demand by creating the illusion of attractiveness to other women by creating a false girlfriend persona through commercial services that allow men to fool others into believing they have girlfriends (the services were not created for the purpose of picking up girls but they have begun to be used in that fashion, much in the way that some girls hire themselves to act as wing-women for PUAs in their area).
Brian
www.BrianDavidPhillips.net