As the other poster said, consider the urinal. Whether intended or not, there are about to be a lot of images of people at the urinal taken by a capture device. Should men just get used to this idea and expect these pictures online?
I roughly get and agree with what you are saying. But, consider, it is trivial to build a microphone that can pick up every word you said within your house. Do you feel that police should be able to use such a device from a van outside without a warrant? Hell, it is trivial to build a camera that can see through most clothes nowdays.
That is, the laws are there precisely to cover things which may be easy otherwise. Hell, it is trivial not to pay your taxes. Illegal. It is trivial not to honor a contract. Illegal. We don't make laws against jumping to the moon. Because... not exactly relevant from a legal perspective.
Yes, or else just walk to the stall and pee in the toilet. I see guys doing that all the time where I work -- some men want to be private about it, and urinals are not and have never been private.
"But, consider, it is trivial to build a microphone that can pick up every word you said within your house. Do you feel that police should be able to use such a device from a van outside without a warrant? Hell, it is trivial to build a camera that can see through most clothes nowdays."
I did say that closing your door should give you a reasonable expectation of privacy. I also said that things that are easy and popular should not be made illegal. If everyone walked around with a parabolic microphone or a millimeter wave scanner, we would have to adjust our laws, habits, and notions of what counts as a reasonable expectation of privacy accordingly. It is currently reasonable to expect that wearing clothes protects you from having your naked body photographed; that would have to change if it were common for people to carry cameras that could see through cotton.
However, if police wish to violate this, using relatively common and accessible tools, they have to have a warrant. Consider the phone calls you place. These are just as exposed to third parties as any email you send. Yet to intercept them police need a warrant. For others to do so is illegal.
Note, the deciding point here is not that it is hard or easy to intercept your communications. Or sit outside with a microphone. The point is that those activities require a warrant. Because they are illegal otherwise.
So let's take that argument to its extreme: you are standing in an open field going to the bathroom. Do you still think you have an reasonable expectation of privacy?
The problem with your argument is that it is based on the idea that if people want privacy, they are entitled to it even if they do nothing to protect that privacy. There is nothing wrong with expecting people to be a bit proactive when it comes to their privacy -- closing doors, drawing their curtains, encrypting their email. People should not just shrug about someone filming them on the toilet, they should do something to prevent it (and if a person started to drill holes in the door, sure, prosecute them -- for destroying someone's property).
"If someone were going into a restroom taking pictures, people would feel rightfully violated"
No, if someone were going into a restroom taking pictures of other people going to the bathroom in a closed stall they would rightly feel violated. Any weaker standard is basically saying that nobody can take pictures in public, because they might accidentally catch a person peeing on a wall and thus violate their privacy.
"The ease with which it can be done is irrelevant to the legality of it."
No, it is very relevant to the legality, because when we make easy and popular things illegal we worsen an already out-of-control criminal justice system. We do not want to start arresting people just because they are using their cameras, even if their cameras are mounted on their heads, even if they wear their head-mounted cameras into bathrooms. If cameras are everywhere and used by everyone, the answer is to build bathroom stalls that go from the floor to the ceiling, not to open the door to waves of prosecution (which will almost certainly be used selectively against "undesirable" people).
The appropriate place to draw the legal line with privacy is with people taking proactive and reasonable measures to be private. Putting letters in envelopes, closing curtains, closing doors, and yes, encrypting emails. You have no expectation of privacy in public parks, nor if your door is left open, nor if your curtains are left open, so why should you expect privacy when you send unencrypted email?