Wow. Thanks again to everyone who has replied. I have a lot of stuff to consider now, and reading the responses has been incredibly useful in gathering my thoughts. I am a bit overwhelmed by the amount of responses but I will do my best to answer as many questions as I can. Sorry I didnt get a chance to reply before but kids have d+v.
Someone said I don't seem surprised. I have strong suspicions that there were things going on in the background pre-marriage, but I naively thought after marriage that would be a red line. So I am surprised in a way, but not shocked to the very core, if that makes sense.
It was 3am and he was drunk, so it is possible that he was investigating/fantasising. I havent uncovered any further evidence.
He has withdrawn from me. We have gone through some huge changes in our life. He put his career to the side temporarily to support me in pursuing my dream job, which I am now doing. This is nothing to do with what he has done, but I am providing a bit of context. Before kids we were very passionate. We have had problems for many months. He doesnt respond physically, and doesnt initiate. So yes, he has very much withdrawn and I see that clearly now.
I wouldn't dream of being unfaithful. It is completely outside what I consider a marriage to be. I thought he felt the same, which is hard part of the realisation of what I have stumbled upon.
I won't be leaving or confronting. I have been being kind and loving to him, because I do feel sorry for him (that makes me sound pathetic) but I don't know how long I can manage for. At this point it isn't really in my interest to get divorced and be separated from my kids. I can't confront him. I feel very weak and lacking in confidence. I feel really sad in the car, I feel empty and dead inside. It is like I dont have a heart anymore. This is the great love of my life, and now I have to cope with knowing this. It is fucking awful.