The Queen (2006)

[Reverend]

Except it's been pretty much the done thing for a royal, especially an heir to have both a wife and a mistress.

Well, there were a lot of things in the old days that are not considered acceptable today. By the time he married, at least in western society, it was frowned upon to cheat in your marriage.

If you go by your logic, by tradition, they should have been able to chop off Diana's head for her adultery.

I didn't say it was right or acceptable by what the general public consider modern standards, just that it's simply the way things were; it was the "done thing." Because the Royals live in such a bubble, they're often caught in a bit of a time warp.