I can't speak to everywhere but it certainly would not have been appropriate for the major kings of the Hundred Years War. That said, it certainly was not uncommon for kings to have mistresses and while some parts of society (mostly clergy and their wives) might not approve it was certainly not considered a big deal. Charles VI was even somewhat famously given a mistress by his wife because his madness had gotten so extreme she didn't want to sleep with him anymore.

David Green's The Hundred Years War A People's History has a decent little section on wives and mistresses that's worth a read. There's probably a more specific book on late medieval prostitution that would be of more direct application to your question but I am, unfortunately, not familiar with one.