It's a story that seems to be going no where is it?
I'm not sure if that's because there is some truth in it or if it's because it's that's nasty kind of gossip that sells papers and gets people talking but the papers are safe as they haven't said anything that can be proven to be untrue!
Whatever William and Kate do now, be it carry on as normal, undertake lots of joint engagements looking happy, have a staged party or engagement with the other couple, can be seen as an admission of guilt. Coming out and acknowledging it as a rumour will also just give the whole thing more steam! They are in a loose loose situation.
If he has done it and it can be proven a) what an arsehole and b) I think it might be the beginning of the end for the royals.
If he hasn't done it then it's a horrible rumour for them as a family hopefully the children are too young still to know anything but still it's selling papers/causing people to click on sites do it's unlikely to be the last!