All cheaters lie. Period. You already know in your gut and your heart that he was already cheating. Yes, it sucks that he is not man enough to admit it, but that's just the way it goes.
When my BF was having his affair he repeatedly lied to my face, denying that he was seeing OW when I had all the details of when/where they were meeting. I just wanted him to admit it and stop the lying but that didn't happen until after we broke up and he decided he wanted me back. Since that scenario is not likely in your sitch I wouldn't hold my breath for him ever 'fessing up.
Plus knowing the details really hurts. I wish I didn't know half the stuff I do, like that he was more concerned with her feelings on my birthday and that he had sex with her in our house.
If you love somebody, set them free. http://tinyurl.com/2empx2g
