So, if one person cheats first and the marriage is broken, the other just has to take it?
(For clarification, I don’t believe that Charles ‘cheated’ first. Diana was as good at PR as Harry’s wife is now - though in her somewhat hysterical way Diana was a lot smarter about a lot of things.)
That person can file for divorce.
After that point they are free to do as they like. As long as the person they are doing it with is also unmarried.
But screwing one of your subordinate's wives is considered a real "HELL NO" in a civilized military. In the US military you will end up with an Article 134, "disorders and neglects to the prejudice of good order and discipline in the armed forces, all conduct of a nature to bring discredit upon the armed forces,". Screwing your brother officer's wives is a good way to have your military lose all respect for you.
I know the idea of at least trying to keep your soul intact by actually acting with honor rather then doing what ever feels good at the moment is sorely at odds with the current common belief but I still think it matters.
Is this really so very hard?