Happened to me too, 20 years ago and it still bothers me. You have to do what is right for you. If you want him to stay then let him but definitely go to counseling. If not, kick him to the curb! Children are better off living with divorced, happy parents than married, miserable ones. We worked through it and are still together but, it always nags me in my mind. Betrayal is difficult to get over if you ever do. Maybe he was afraid you would not want him anymore now that you are a new, beautiful you and he needed to seek validation elsewhere. He may have needed to see if he was attractive too.
Here is something I have just recently learned and I am going to be 50 in 6 days.....forgive him. I'm still working on it obviously.If you forgive him it puts all of the onus on him. You will release yourself emotionally from the burden and he wiill have to carry ALL of it. Forgiving does NOT mean forgetting or becoming a doormat, it's just saying you did the wrong now you have to deal with it because I will not pollute my soul by carrying this ugly thing around within me every day...I am done with it because I forgive you but you have to live with it every day now, you did the wrong, you suffer, not me.
I always thought forgiving was absolving the other person of the wrongdoing and that I would never forgive but it's really about letting the ugliness go and realizing you had nothing to do with it. It's a gift to yourself.