I've encountered this sort of situation twice, first was on a local off road route, where there is a short narrow bridge over a ditch. Open on one side, it has a steel handrail on the other, and I caught a couple at it, the guy's shorts around his knees, the woman resting against the handrail, and what I have never figured out is whether the words she uttered as I rode past ("Oh goddddd.....") were her pleasure, or embarrasment at being caught. Definaetly think that was an affair type situaion, I recognised the guy as a local private hire driver, the woman appeared more smartly dressed, although I suppose money could have changed hands! The bloke appeared more concerned about the risk of being skewered by my bar end!
The second was on a lane, sadly quite close to some houses, and an older car was parked up in a stupid manner. As I rode past there was a stench of perfume and sounds of passion emanating from the open rear door window. That was about teatime on a midweek day, and again I think that was probably extra-marital.