@taybert
Gossip about lawful private lives shouldn’t be a matter of public interest.
Sorry but this is very naive. It's always been exceptionally hard to define "public interest", as opposed to "that which the public is interested in". But if you seek to make speculation about people's private lives off limits as a matter of course, you risk restricting people's legitimate right to challenge the official narrative which they are given and that's a problem for democracy.
At the most narrow definition yes it may not strictly be in the public interest for people to know that HRH Prince William likes taking it up the khyber. But where do you draw the line? Do you say that any reporting on or speculating on the private lives of the royal family is off limits?
Had you done that the scandalous treatment of Princess Diana by Prince Charles and the Royal Family in general would never have come to light. There are plenty of other cases in history, such as the Profumo Affair etc, which would also have been made illegal to report or comment upon if there had been rules about not reporting on people's private lives but which actually had much broader significance. It's then a slippery slope for politicians, royals or other public figures to seek to prevent people reporting on these things.
The line between grubby tittle-tattle and public interest isn't always as clear cut as a lot of people like to suggest. The numerous affairs which Charles and Diana had during their marriage were on the face of it not matters of huge public interest but they did have profound constitutional significance. Bill Clinton's affair with Monica Lewinsky is another example. On the face of it it was "just" an affair but actually it raised profound questions about his trustworthiness and competence. I'm not saying that anyone who has an affair is automatically untrustworthy in their public role but it points to a capacity to deceive and surely something which members of the public have a right to know about.
Finally, the HRH Cambridges are in a position of considerable authority and power. I have a lot of sympathy for them and I wouldn't wish their lives on my worst enemy but you have to allow scrutiny of people in such positions of power.