Things that are called normal, are the things that are gratefully more common. For example, most people drive forward on the road, that is what the majority does, so we call it normal. If someone drives backwards, well that is possible, but it's not normal. But I see you're conflicting calling something normal or abnormal, with, calling something acceptable or unacceptable, or existing or non existing. Just like a driver driving backwards on the road, that is completely acceptable, but it isn't normal.
So, how should I call it then if the majority of teenagers use Dad and Father ? How should I call that type of action if I can't use the word normal ?