This like the .Friends episode where .Ross and .Rachel were on a break and she kept saying he had cheated on her and he kept saying, "we were on a break!!!"
apparently he loves you, he is back with you. You two are having a child together. Your only option now is either to forgive him or not. If you don't, you may as well not even be with him because it will only cause bitterness.
As far as him 'deserving' you sleeping with someone else...I find that to be immature thinking. You made the choice to sleep with someone else to hurt his feelings because he slept with someone else and hurt yours? How did that work for you?
Good luck. A lot of the anger and hurt you may be feeling at this moment might be hormones and thankfully those get easier to deal with in a few months. Sit down and talk with him. Be honest, encourage his honesty. I hope you can work this all out.