Why should you wash your hands whenever you’ve just used the bathroom? (Don’t worry, I do. Explanation below. No need to stop shaking hands with me!)
There’s plenty of evidence that washing your hands on a regular basis has health benefits. But why do it after using the bathroom, specifically? Because you’ve been touching yourself in your private places? The very fact that those places are private is reason not to worry much about them. Your hands are out in the open all day, touching surfaces, pushing buttons, shaking other hands. Your private parts are usually covered with at least two layers of clothing, well shielded from outside world. If anything, the danger is not that your private parts will transmit germs to your hands, but the reverse: your hands will transmit germs to your private parts. This is especially true given that your private parts, unlike your hands, have mucous membranes. Wouldn’t it make more sense to wash your hands before using the bathroom?
I hear the response: no one wants to touch your hand after you’ve been touching your business. But why not? If they’re already shaking your hand, they’re already being exposed to something more likely to be contaminated than anything under your boxers. Of course, it’s possible you could get actual excreta on your hands during the process. If that happens, though, you generally notice and wash your hands anyway. This doesn’t explain the rule saying to wash your hands every time.
Maybe you’re likely to touch surfaces in the bathroom. Okay, but why are those surfaces any worse than any of the other surfaces you touch all day? Because people who’ve just touched their privates touch the bathroom surfaces as well? See above. Also, according to at least one study, phones and keyboards have more germs per square inch than toilet seats.
I think the real answer is just that the bathroom rule is easy to remember. It’s a good idea to wash your hands regularly, and the bathroom is one place you visit regularly that always has a sink. Whatever else you happen to do there is, I suspect, mostly irrelevant.