The passenger in the vehicle passing them was so shocked he got his mobile out and recorded them in the throes of passion.
The male driver is seen somehow still steering while a woman bounces up and down on his lap.
But when he realises he's being filmed, rather than slowing down or pulling over, he sticks his middle finger up on his left hand as the other driver passed.
The uninhibited pair even appeared to have turned their hazard lights on by accident as they appear to make love partly-dressed.
Local reports said the car was doing around 70mph.
The shocking incident happened near Yala in the province of Jujuy in north-west Argentina on National Route 9, a major road which links the capital Buenos Aires to the country's border with Bolivia and crosses seven provinces.
Argentina has one of the highest traffic-related death rates in the world, with around 22 people dying every day in road traffic accidents.
Argentina topped a road traffic death league of shame released two years ago by the International Transport Forum, showing more than 5,000 deaths per year, although the situation at the time was said to be improving.
Parts of Route 9 have been branded among the most dangerous in the country, with animals on the road being a problem as well as the number of people who still drink-drive.
Ten vehicles including five lorries collided on the road last month during a dust storm, causing one death.
Local newspaper Notinor Jujuy, which first published the video, said: "This is a 'No-no' .
"A reader filmed this foolish behaviour by a driver who was on National Route 9 near Yala this weekend.
"Roads in Jujuy have a high accident rate and this type of behaviour definitely doesn't help."
Police are understood to have been sent the footage so they can investigate, although the driver's numberplate doesn't appear.
Writing on social media, local woman Vanina Nancy said: "It would be your problem if you crashed but why put the lives of innocent people in danger.
"I hope your family recognise you in the video."
May Villalba added: "People with no brains. Maybe these idiots don't give a damn about their own lives but there are people on this road who do and make sure they do everything right from the moment they put their seatbelts on."