If-as Diana and her friends always insisted-Charles was romantically involved with the very married Mrs. Parker Bowles up to the day his engagement was announced he IS the one who cheated first. And he duped an immature teenager into believing he could love her, which was even worse.
Perhaps after the marriage collapsed Charles and Diana believed that the centuries old maxim that allows upper crust couples to have discreet extra-marital affairs also applied to them. And the status quo might have been successfully maintained to this day if Diana had not decided to give a half-baked version of the truth in a tell-all book.
Some of what you're saying here is putting words into other's mouths. We don't know that Charles set out to dupe Diana into anything - not having been a part of any of the conversations we don't know whether Charles acted like he could love her or not. In fact, given as he was rather public on the fact that he wasn't exactly sure what love meant prior to his marriage (but after his engagement) I'm not entirely sure how Diana was duped into believing he was in love.
That said, if Charles' relationship with Camilla ended before the engagement I'm a bit confused as to how he cheated then. It's not exactly like he and Diana had any courtship before their engagement. Can you really say the, what, 6 months before the engagement was announced counts as him cheating? He continued to have a friendship and sexual relationship with a woman who he had no intention of marrying, who had no expectations of marrying him, and who was married to another man, while he began to develop a relationship with a woman who he considered as a possible wife. When he then decided to marry her he ended his other relationship... sure it's not exactly the cleanest of starts, but we're literally talking about a 6 month period here.
Charles then didn't resume his relationship with Camilla until he felt his marriage was essentially over. This is after years of a lot of problems, including his pregnant wife throwing herself down a flight of stairs. Diana is being accused by her former lover of having started an affair just two years into the marriage. That is her apparently cheating first - although, she may well have felt that the marriage was pretty much over at that point, I don't know.
That was my understanding as well. Of course I am applying double standards: "Gentlemen may cheat, ladies not".
This is rubbish but my politically very incorrect gut feeling says that the Princess of Wales, a young mother also, should have shown the class her spouse apparently lacked. But she did just exactly the same. The Princess of Wales and future Queen of the United Kingdom made herself available for an army Capitain. And that same officier knew he was dating a married woman, married even to the person who once would become his supreme commander....
If Diana started having an affair in 1983, then she was showing a lack of class that her husband seemed to still have, given as he didn't start his affair until 1986.
I don't think Charles is any worse than Diana for cheating, nor Diana any worse than Charles for having done the same, but saying that one is displaying the lack of class that the other had should at least consider the apparent timeline here.