Well, Jorath, girls in a relationship feel more secure, so are more likely to flirt around. Believe me, I am not stranger to the feeling that "all the good ones are taken", or as a girlfriend of my wife's says, "Dating now is like a mall parking lot at Christmas: all the good ones are taken and all that's left are Handicapped." Don't worry, these great girls, or some of them, will get tired of their boyfriends and you can swoop on in if you have stayed in touch. Believe me, it worked for me.
