I am sorry you are having to go through this...yes it is cheating

I feel if he is unwilling to respect your feelings and earn your trust back then you should talk to a divorce attorney and find out what your options are. I feel that he definitely has something more to hide. And if he does not feel that chatting online is cheating then chances are he will do it again with someone new. That means you will have no peace in your life. This guy doesn't deserve you if he can't respect you.