Wow, that is great and I am very happy that you got it out in the air and out of the closet knowing how shock you found out what been going on between your Dad and Mom in the past. Boy, I wish I could have ask my Mom and my Dad about what they have felt about me being deaf when I was a newborn baby and growing up. Problem is they went to the grave with their secret thoughts. So I was not lucky, just to have a moment to find out. Like your Dad said that if the hearing person don't use sign language, then this person is really nuts. I really hate having the parents listen to the "experts" like doctors and audiologists plus oralists to tell them that Deaf/deaf/hard of hearing should speak. We really need to educate them and letting them know we really need visual ASL. Gawd, don't they get it every time we try to tell them. They just won't listen. It is very exhausting listening to the hearing people who are againest ASL. I wish many hearing people will read your post. 
