As jwenting said, it's unlikely to happen for many reasons, and if it does, the participants are even less likely to admit it since doing so could get them in trouble both professionally, and with a government regulatory agency for endangering safety. I seem to remember the FAA taking action against a pilot who posted photographs of himself flying with either naked or bikini clad women at his side, but I can't seem to find a link right now.
There is one NTSB report which is somewhat famous in the aviation community for describing a general aviation crash which was likely caused by sexual activity. However, the summary is very delicately worded, to say the least:
The private pilot and a pilot rated passenger were going to practice simulated instrument flight. Witnesses observed the airplane's right wing fail in a dive and crash. Examination of the wreckage and bodies revealed that both occupants were partially clothed and the front right seat was in the full aft reclining position. Neither body showed evidence of seatbelts or shoulder harnesses being worn. Examination of the individuals' clothing revealed no evidence of ripping or distress to the zippers and belts.