Fiance talking to girls behind my back again, what do I do?

We've been together for 4 yrs and have two little girls! Our relationship has been great until I came across him sending pix of himself and leaving girls comments on their pictures like "you're beautiful, yummy, ooo now gives u kisses on you neck...etc.". He's also told a girl not to message him on myspace cause I check it! Mind you when I confronted him on it he denied it then lied. All he can say is "sorry" and "i would never cheat on you". I dont think I could trust him again. I am so hurt and confused as to what to do... cause this is not the first time he has done this, I can think of several other times. All I can think of his my girls and whats best for them, but If they werent in the picture I would have left him long ago! I need some advice... Girls and guys... Help me out.