Why do girls flirt with guys that they know are taken?

[Flappers Wing]

My boyfriend has many girls who like him, and they all know about me. He's friends with some of them on facebook and I'm in his profile picture, there are several pictures of us (some kissing) and the obvious: it says he's in a relationship with me. Yet some girl he coaches for a basketball team sent him dirty messages (He deleted her). I've met several of them. They continue to flirt.

Now this isn't about me being jealous. I try very hard not to be, and I know I'm his only one. He makes sure everyone knows about me, and I trust him. What this is about, is these girls... Why do they keep coming onto him even knowing he's in a committed relationship to me? I thought the rule was to back off your crush when they get in a relationship, at least that's what I've always done - as soon as my crush got a girlfriend I got over him. We've been together for about 5 months and girls he's known longer than me still flirt and try to get him to do stuff with them. It's like they don't care, or they totally disregard me, like they are encouraging cheating behavior from him. How would they like it if this was happening to THEIR boyfriend?

Why do they do this? Would they be more respectful if I were actually friends with them and we knew each other? Does this happen because they don't have much to lose, barely knowing me? I'm not like this, all girls can't be like this... please tell me this is just an unlucky streak of disrepectful girls he is getting?