I can't speak for your husband but speaking as someone who has experienced hypomania I can say that I absolutely knew what I was doing all the time. The question was whether I cares or not that it was wrong. I agree with the above poster - I would feel that I could not be touched and could do whatever I wanted because I was so awesome and no one else mattered. The only exception is when I became psychotic from my mania and believed someone was implanting thoughts in my head that I had to obey. That time I felt I had no control over my actions.

I have not cheated or even come close or even talked to another man in that capacity but when I am hypo I am MUCH more social.

So he may really have not realized what he was doing was cheating. Or thought that it wouldn't matter. But having bipolar is not an excuse for poor behavior. As long as he is accepting treatment for his condition it should not happen again. Work with him on identifying episodes so they can be caught before they get out of control. That way you can try to avoid another instance of cheating.