I think the main difference between not liking an old man x young woman couple and liking one is the old man. If he looks like an ugly bastard and acts like an ugly bastard, no one likes it. If he's an adorable marshmallow in human form, then everyone likes it.
I hope the art's drop in quality is temporary. The series has never really had amazing art, so it dropping any more than this would make it almost unreadable
isn't this basically NTR? If the hero is going to cuck the demon king, at least make sure she consents and don't do it right in front of him. or just don't cuck someone's lover in general.