Just to get something clear.

I am NOT okay with SO sleeping with another girl. We had an agreement that it would not happen. If it where to happen I had to be there and take part so there would not be an jealousy issue (we had talked about this, and we both agreed on this). He was free to spank others and some other things, but not anything involving sex. I trusted him, and didn't ever believed he would go behind my back. The way I see it, it's like "normal" infidelity. Our limits may have been wider than some others (some may set the limit with dancing with another, some with hugging, some with watching porn and some may bee free to do whatever they want, etc.), but it was still a limit, and he broke it and went behind my back and lied.

First, he was very defensive when we talked about it, and I was angry. I felt he said all the wrong things.
When at couples therapy I learned that he really was ashamed when he was defensive, and he learned how it felt to me when he reacted that way. He accepted that he should not have a "private" life for a while, and now he is encouraging me to check his emails, cellphone etc. (and I don't feel the need to do so that much, since he isn't bothered with it, so I guess things are stabilizing.)
He have agreed to not do anything with other women, no spanking, no flirting on Facebook, no suspension and all those things I didn't bother with before. And he knows that I neither will do anything with other woman as long as we are together.

When it comes to our sexlife, there is at the moment almost no sexlife. We have agreed to start fresh step by step, little by little. I really need to find my limits after neglecting them for years, and I guess it wont happen very fast. But he is very supportive now, and I have a feeling we will be okey in the end.