Consequences of getting caught during intercourse in a car?

Frankly, younger folk get caught all the time, but they're more often doing such things out of necessity -- they can't go home or afford hotel rooms, for example. If you're older and married, it's going to be more obvious you're doing it for the thrill, and not out of necessity. That might not go over as well, and make you less likely to just get a warning.

You're indeed looking at public lewdness charges depending on details. The police are likely to be harsher if you're easily visible to others or in a very public area, or if somebody else has had to call to complain. Otherwise if caught, they may or may not let you off with just a warning. And for what it's worth, I'd keep in mind that even though what you're doing may be consensual, a lot of acts like this in public locations are often assaults of some kind, so people (and cops) very often don't look the other way if it's encountered.

As far as being on farmland, yes, they might arrest you, as you'd outright be on somebody else's property, so there are also issues like trespassing. If somebody's encountering you there, it means they've seen you, or somebody's complained, etc.

Where I grew up, there was a secluded on-lane gravel lane next to the river. Nobody ever went there, except the fire department when they wanted to fill up the pumper's tanks, and the occasional pair of amorous teens.

Well, and the local police, who would run the occasional night-time patrol.

What they'd usually do is flip on the headlights at close range, wait for everyone to get dressed, and inform the occupants that if they didn't move the car, they'd get a ticket for illegal parking (blocking a roadway). They would sometimes follow that with a warning to the female that women had been known to be raped on that stretch of road.

I'm not sure people would be let off with a parking ticket warning these days.

Public lewdness/Indecent exposure depending on state. Punishments vary by state. It's always a misdemeanor crime.

It's hard to predict what each officer might do. Chances are you'd be cited.