I don't think anyone here is suggesting for one minute that you are the one sleeping around, in fact, you -like his girlfriend are the innocent victims, in a sense.
I suppose, based on morals you should have told him to end his relationship if he intends to make a go of it with you before 'anything' happend. But life's not like that.
He's basically got you for a bit of fun - anything he says and does may come across as a strong emotioinal connection but the reality is, he'll say anything to keep you interested and while you're emotions are running high, you'll believe him.
This is going to sound realy harsh but, you might not be the only one he's connecting with behind his girlfriend's back.
Be careful.