You are getting good advice here. Confronting the truth about someone you love and thought loved you can be devastating and frightening. Many of us equate love with fidelity and assume if someone loves us that they will be faithful. As you can see on this web-site, that is not always true. There are close to 40,000 people that have used this website.
Your husband is probably a sex-addict and the last thing a SA wants to do is admit that. That is why he is so defensive. He has separated his need from his marriage and thinks that his "hobby" should not have anything to do with his and your relationship. Called compartmentalizing. He is deep, deep, in the fog, and the sex to him is like a drug. It takes just as much work and pain to kick that habit as it does cocain or heroin.
You need to give him a real hard dose of tough love. He starts counseling or you are gone. Go see and attorney and get your financial ducks in a row. Get some money put aside and make a plan as to living arrangments, the kids if any, and let him know you have done that.
If he resists counseling, start a really hard 180. You need to be in counseling also. Sometimes you can use the same counselor for IC and MC. Depends on the counselor.
We do and it works well. We go separately sometimes and together sometimes. Our counselor gets to hear from both of us and I am sure it better helps her to understand the dynamics of our relationship.
He will continue this behavior until he is forced to stop, either by shame or by the terror of losing you.
If he lets you go without trying to keep you, then you are better off without him. Your future will be one painful revelation after another.
So, so, sorry he has brought this pain upon you, but this is a great place to vent, cry, and, heal. Love and hugs. K