Most Helpful Guy
Well, you trusted him and had no reason to suspect he was cheating. Having trust is a good thing. I think it's better than the alternative; being suspicious of every motive your new Boyfriend has. It's exhausting and you are not trusting the person your are with. And without trust, then what's a point?
Sorry to hear that he betrayed that trust, but you should've known his motives from communication about your relationship. If he was clear about "having fun with" you then you should think twice about moving in if not on the same page.
If he lied, then to hell with him!