Why not? It is obviously a problem that needs to be solved and that might be difficult to solve, but NASA has always kept quiet about what the solutions are, so people are curious. They're curious but they are afraid to ask about it. Prince Philip isn't. Why does it have to be such a big secret? One doesn't necessarily have to be uncouth to describe the solution. Although the Duke of Edinburgh may not be the brightest bulb, I don't think his asking this particular question provides evidence of a lack of intelligence or propriety.
BTW I once saw Prince Philip quite close up at exactly the place you would expect, a polo game.
My point about not asking the question is that the answer is pretty obvious. Not to mention that it was gone into detail when Nowak took her trip across the country. Many assumed she was wearing a NASA diaper, when in fact she probably just used the Depends brand.