If you're not an avid reader of Tatler, the publication Ian Hislop calls a "magazine for k--bs", a recent column on sexual faux pas probably slipped under your radar.
But if you were curious about the etiquette that governs the romantic lives of the upper crust (or 'shagging', as the author calls it), according to Tatler, acceptable bedroom conquests are:
Fine, just don't tell anyone.
Also fine, but be careful about the in-breeding.
Your brother's wife
Only if he is dead.
Your son's friend
OK as long as you're divorced, and he's handsome and over 30.
Your best friend's ex
This is maybe OK if she's now happily married.
Allowed if you're over 25, he's single, and you have a healthy relationship with your dad.
Turns out posh people draw the line at animals. Although maybe the rules are different if the animal's dead?