It seems I may be late discovering this thread but wanted to toss out some stuff in the event it's still active and you want more feedback.
There's a lot of reasons men (or women) cheat but for the most part there seem to be about four primary ones. I suspect you are probably aware of these so nothing new here, but I think it's worth laying out and trying to address logically. The reason I feel this way is, dependent on what you discover from the exercise, it will help chart the course for the future.
1> Sexual chemistry diminished or needs/fantasies not being fulfilled
So many people hook up (marry etc) based on NRE, and often there are hopes for what look like missing pieces to magically show up later. Usually doesn't happen. Add to that kids entering the picture, a slowing down of passion, body changes etc etc and the relationship has grown to another stage. They ALL do !
Some can't get a grip on that, want to go back to the first 5 dates. Not happening.
Is this where he is at ?
2>Emotional distance and thinking sex will fill that hole
Sometimes, as relationship go through all the various up & downs and phases they do, it seems we get on a different path than our mate/loved one. We feel that distance, don't (without study) understand it, feel it's 'gone to hell' and end up open to something new to fill in where we think we're missing. Sex is the most common attractor, but in reality it's usually something else that we're truly missing. But we go for the sex believing the rest will come with it. Another false belief
Is THIS where he's been at ?
3>High sex drive and ongoing curiousity about experiencing others sexually
For a lot of men, there's just too much testosterone flowing and the whole world looks like a sexual playground. There's always this curiosity on seeing someone attractive about how they would be together, what new thrills the other person might bring to the table etc. Some like to classify this a 'sex addiction' but I think that's less than accurate. But if you have this type of person in a relationship that passed through the NRE/exploration phase, it's usually just a question of time. This needs to just be acknowledged and if it is , it CAN be worked through. It's not really an aberration - just needs some safe and realistic guidelines put around it.
Is THIS where your DH is at ?
4>General dissatisfaction with the current relationship
Sometimes it happens ! People marry and discover, once the masks are all off, that who they married is not who they thought they were. And sometimes people change - dramatically - and are no longer who they were and don't want to be. It's real. It happens. If it does, the search starts for someone new and of course sex will just be a natural part of that.
This is the hardest one because in most cases there's no way forward from here together. You just have to call a spade a spade and move on as gracefully as possible.
Based on your writing as well as the one post your DH tossed in, I don't 'think' this is the case, but it still needs to be put out on the table and analyzed as a real possibility.
So, I think you both need to figure out which one (or more) of these best explains his behavior. What you find out will kind of set the sights (and options) for the future.
It's not that hard. Just strive for honesty. The masks got you into this mess, time for them to ALL come off.