E.T.
M., I just went through something very similar. My husband signed up for online message boards and adult dating services and was messaging with women about getting together. I don't know if he ever really slept with anyone (he says he didn't; sometimes I believe him sometimes I don't). What bothered me more than potential sex with other people was the lying.
Every situation is different and however you react is the way you're supposed to. You may decide to try separation, go to counseling, divorce immediately, forgive him.... so many different things you could do. Only you know what's right for your situation. A book that helped me was Too Good to Leave, Too Bad to Stay.
If you want to message me, I'd be happy to tell you more about how I handled things and why. In the end, I decided to get divorced and it was just finalized this week. The most important thing you can do is find someone to talk to about this - whether it's people online, a friend you trust in person, or a counselor/psychologist. You need to wrap your head around the reality of your marriage versus the way you thought it was. And talking it out with someone and getting legal advice if you decide to separate/divorce is the best thing you can do.