Had it happen a few times during my 135 Lear days! Got a nice tip out of it once!!
Of course nobody chartered the Lear strictly to join the mile high club, actually it was kind of a pain in the a$$, having someone rocking the in back of the Lear at FL410 with the JET auto pilot.
The funniest thing was when one of the girls poked her head through the curtain, and started asking questions ("what's that button do, what's that"); trying to act all casual while the guy is going to town on her on the other side of the curtain.
Then there was this one big customer, we actually had to call the FBO using code words to make sure his wife wasn't there waiting.... we actually had to taxi into another FBO once and drop off the female passengers (more than one), before we could continue on to home base FBO and the waiting wife!
Then there's the putting your foot in your mouth category. I asked this passenger, how she and her husband liked the trip to Aspen two weeks ago, unfortunately I didn't take the wife to Aspen a few weeks ago, only the husband and a different blond!!
Hey, I'm happily married and I didn't agree with it, but you know what, it ain't my d@mn business!