I don't believe Diana would have ever cheated if Charles had loved her and remained faithful to her.
We won't ever know, but the fact that she herself conducted an affair with a married man whose wife felt betrayed shows me that she didn't think that highly of marriage and faithfulness. It`'s strange but I think the affairs with Charles and married women pre-wedding happened because it was for him a safe opportunity, for the women it was part of their society live. If your marriage cooled down and yoh had given your husband an heir, you could do as you liked if you were discreet and the man was not married himself or he and his wife followed the same codex. I'm not aware that Charles had affairs after his marriage, except for Camilla, whose marriage was already on the point where APB had his own affairs, so was in a way "free" to have an affair. While Charles only started with Camilla when his marriage was really broken down. And both stayed faithful to each other ever since.