I think divorce is the last step - and a regretful one. Strongly recommend you try and bring him to your point of view. You may be very surprised (I know I was) to discover that he likes the idea as well!
Divorce is definitely the last step, but you are already heading down that path if you are cheating IMO. I am not judging, it is simply my perspective on that particular behavior.
I had all the fears associated with the wife sleeping with other men, but I let the excitement of it all outweigh my fears. She probably manipulated me into it. Women are smart like that when it comes to men I believe. I try to never underestimate her ability to manipulate me, but it probalby still happens. She was reading "smut" and the resulting sex was awesome. When I asked her about it, she "reluctantly" told me that her favorite subject was to read about threesomes. It became a common topic during sex, and the next thing you know i was asking her if she wanted to add another guy to the bedroom. She of course vehemently denied an interest in that (at first), lol.
The biggest surprise for me was that my wife loved for me to be excited by it all too. I thought (and feared) she would just be thinking of, and only into the guy she was with. So when I started saying things like "doesn't my wife's pussy feel good", etc., she clearly loved it. When I started using my hands, mouth and dick to compliment what her "friend" was doing, her orgasms were amazing. She thought squirting was a myth until she was riding a well-endowed young friend of hers, and I began playing with her clit and ass at the same time. She became a fountain much to all of our surprise.
So there is definitely lots of fun to be had if you can open him up to it. If he as absolutely never considered such a thing, even in his own head, the transition could be rough. If he is a perv like the majority of us, it may be a little easier with the right moves as long as you don't scare him off in the process.