Guy I like has gf and is a total flirt...help?

I think you should probably stay away from him. He might just be up for casual flirting but I don't think taking it to the next level is on his mind. You don't want to get hurt (by him) or end up hurting others (his girlfriend). It's okay to casually flirt back with him, after all flirting is normal, but you shouldn't expect anything more from this guy.