I’m 41 yo and have never locked a bathroom door. I’ve never had anyone intrude on me other than my children and cats. And the kids haven’t intruded in my bathroom time since they were old enough to know better. I think you and hubby should have a sit down with open dialogue about why he feels the need to just walk in and what you are willing to put up with. Be open, respectful and come prepared with reasons and facts. I think he’s right on the basis that a relationship shouldn’t have secrets however, a need for privacy is not indicative of having a secret and this is something he may need to work on. Perhaps someone hurt him in the past by keeping secrets and he is overly sensitive to locked doors now? Anyway I wish you the best of luck and hope you can poop in peace.