It is not just anyone who would do it.
There is no comparison to Kate and Camilla in this context. Charles was not in a committed or monogamous relationship with Camilla in the 70s. They only got together as lovers and fell more in love after his marriage to Diana "irretrievably broke down." He dated all kinds of women between Camilla's marriage to APB and his own marriage to Diana. William's situation is completely different. William dated some different women in the university years, but then after xmas2003, or thereabouts, it was Kate, and they came out with their relationship with that ski trip in April 2004, and it's been Kate ever since, except for that very brief "cooling off period" in 2007. Kate never married someone else; William has not dated other women, despite the media's attempt to make Jecca into a rival for Kate. So I don't see a Camilla comparison in this context at all. Now if Kate goes off and marries.... van Straubenzee, and then William scoops up Jecca or someone else, marries her, and then gets together with Kate adulterously.... now that would be weird! lol