Don't know if he's cheating or not...

I have been dating this guy for a few years. Recently he had been telling me about a girl that calls him a few times a month just to say hi- then I found out that they have been speaking for our whole relationship, and he's been speaking to a few other girls that I had no idea about. When I confronted him, he first lied, then started slowly admitting to it. Now he says he's sorry but has never cheated on me, and I'm the girl he wants to marry. We have so much fun together, and I really can't imagine him cheating on me, but I don't know what else to think..