One possibility is that when seeing a gf, you may not be actively looking for 'it' (be that sex or relationship), which may make them curious as to why you aren't showing any interest in them as in 'i feel good because this that and the other guy flirt/pay attention with/to me every so often, life is kinda rosy, hey wait, what? why is he not also paying some attention? hmmm, whats with him, hey, hey, HEY! thats right, girl, over here, hello, good, attention gained move on"; sort of like paying ontological tribute to someone, its worth more if there is a significant other (might be wrong on that bit) and if it is not given freely, sometimes it has to be demanded.....
when without a gf, lacking the social skills and nuances of interaction that we do, they see that we are/may be looking for 'it' and don't consider the 'tribute' worth acquiring when that encouragement may lead to you taking it as meaning a little more than it is
hmmm, not sure if i make sense to myself......