Talk with your husband. Communicate. Let him know you know. Ask him why and his feelings. Marriages fail because of lack of communication. Think it's in France where everyone is expected to have an affair.
Don't let puritan sex culture ruin your love and marriage. If you can have an honest conversation and communication you can actually build a stronger relationship/foundation and trust between each other. You shouldn't feel angry but hurt that he couldn't be honest and talk with him about it all. How to fix it going forward or not. It's not the sex with someone else that you should let bother you. That's just human biology and sometimes just a stupid mistake/break from reality to de stress. It's the fact that he broke the trust and communication with you. If he is serious about you and the marriage he will be sorry that he didn't tell you he did it or that he never communicated how he felt. If you believe that together you can make communication better than before and he knows how much it wasn't worth it and it will only be a monogamous relationship, then work it out with him. However if he blames you for why he did it or doesn't show remorse or extreme desire to improve communication and trust then ya.
You both carried this stress n weight needlessly. It effects your health. You both should be trying to make each other's life easier and happier. There's an elephant in the room that I'm sure both of you are afraid to address.

I am not upset by sexual desires or let someone try to humiliate/degrade me by saying they got to fuck my wife. I get her all the time, you were a mistake. I was upset because we have always been open books and completely truthful with each other yet she still did it. And her naivety is what got her into the position in the first place but I know if I was in her situation, I'd do the same thing. I just wish she was honest from the get go. It was rough for a few weeks after she finally told me months later shortly before our wedding. I remember that night that she did it she came home late and I was mad at her and she cried her eyes out. Turns out she was crying cause of what she did. She knew she hurt me and was honestly devastated by her choice. She really wanted to be with me. We created an even stronger foundation of trust and communication as a result.

Him having sex with someone else is not what should be the reason to upset you. It's that he can't be open and communicate and you can't either with him. (Unless you fear for your life or baby's but then that's a totally different thing all together)He broke your trust with him and risks degrading how important you are to him. If you can't fix the communication and explain your feelings entirely to each other, him having an affair is not the cause but rather the catalyst to end it. Whatever you guys agree to do going forward to make your marriage/lives happy again is your guys decision and only yours to make. And don't let anyone try to make you feel bad/humiliated by your decision if it's what works and makes you happy/what you want.

And to anyone saying as a guy I am just promoting male chauvinism or trying to blame her/say he's just being a guy, it's the opposite. The shoe could be on the other foot and the advice still stands for me. Life is full of shitty days and stress, don't create artificial bull shit to make someone else have a bad day. Do a little extra effort if it makes the next guy have even a slightly easier time, they could be having a real bad life and any relief is welcome.