I think that if it was proven to be true, you would have to wonder at the 'source' of lots of those rumours! And yes a despicable thing to do to someone! Especially when there is/was enough horrible press coming from the Markle side of things (I'm talking about her Dad and Sister not Meghan herself to be clear).
I think again if proven it would be hard scandal to come back from! It took a lot of PR to bring it back for Charles and Camillia and that is still ongoing. In their favour with hindsight it's obvious Charles and Diana weren't suited and married to quickly! Also and maybe I'm wrong Charles and Camillia seem to be in love and I don't think there has been a rumour of Charles playing away or any kind of affair he has had apart from Camillia, so people seem to be able to accept the idea that it was all in the name of love and it was poor advice bad decisions that caused it all in the first place. Most people don't like it but the will accept it so it seems anyway.
If and of course it's still a massive if it's proven William is playing away none of that would hold true. William and Kate had a long courtship had lots of time to get to know each other, there has been no reported pressure from the palace as to who He should marry and worse of all this doesn't seem to be a case of falling in love if the rumours are true it seems to be a case of a casual affair I think people will view that differently and it will be the end of the royals. Once the queen goes without the popularity of the future generation (William) I think it will be the beginning of the end.
The thing is I'm not sure how these rumours will ever be proved or disproved? Is there anything that could be done to shut them down conclusively?