It is very different if the OW does not know that he has a wife, maybe he said he is divorced
The OW in my marriage - no she doesn't and didn't give a damn for the distress caused to the rest of the family. Yes, she knew we were married, had known for 20 years, had been my daughter's guide leader, and had had both children in Sunday school. I can't imagine how she thought it was at all acceptable. Shouldn't imagine Camilla thought about how Diana would feel.
I sort of get that in the days when marriages were arranged, as with Edward 7th, and it was acceptable to take a mistress as long as you kept her in the background and the wife knew that might happen from the start, and you let your wife do more or less what she wants [ apart from take a lover I guess] things were different, the princess/queen didn't need to worry about bringing up the children on her own [well a royal family member would have had servants to help] or being thrown into poverty when you were divorced because the husband won't pay for the children etc
Although C and D wasn't an arranged marriage per se,it was a marriage of convenience but D didn't realise it until too late. She should have said sod the teatowels, and bailed but she was too young to have the courage to do that, she was trapped