What do you do when you find an email from your husband to another couple for sex?

I found an email that my husband sent to a couple who had a post in the personals on Craigslist. The ad read: MW for M- a couple wanting a man to join for some bedroom fun! His reply read: single 34yr old guy that has always wanted to be with a couple once. I would love to suck his, while she sucks mine, then do whatever you want. Lets get together and beep! You host. I can't. Let me know. I am very clean and very discreet. Till then. He claims it was just a fantasy and I should forgive him. We have 2 children, and I do not feel I have any choice but to leave my marriage I can never trust him again, he has cheated on me once before we ever got married and got help for that and finally I was ready to trust him and marry him and now I am so broken and have nothing left to give! How can there be anything left to save?