Why do guys who have girlfriends pursue other women?

My guy friend is in a pretty serious relationship, but keeps hitting on me. If he doesn't want to be with his girlfriend, then why is he with her? And since he's choosing to stay with her, then why does he feel the need to pursue me at the same time? Any input is welcome!
Updates
+1 y
(Don't worry, I am NOT accusing all men of being like this, NOR am I saying that girls don't do the exact same thing sometimes... I just don't understand WHY. No need to get defensive).
Why do guys who have girlfriends pursue other women?
Post Opinion