This sort of thing is very common in Japan as almost all young people live at home with their partents until they are married. With the marriage age increasing you quite ofter get 30 year olds still living at home with mom and dad. Combine that with the fact that most family homes are very small (ie. no basement to sneak away and have fun in) there is really no other choice but to do the old car shag.
There is another possibility and that is what they call a love hotel. These are much like motels back in the US only they are geared towards the short time stay people who pop in and pop one and then are off again. The can be a little expensive to use all the time (50$ short time) so the car is the best bet.
Add to the scenerio that infedelity is very common in Japan and you have people sneaking off all over the place for a quick shag.
I usually go for a ride after work along the ocean front and see many many cars parked in secluded areas, and you know what is going on in them. Couldn't imagine snapping pics but it would be rather amusing. The looks on some of those people's faces is priceless. Not only are they being caught shagging but more often than not they are cheating too. There is evidence out there on their shananigans.
Another thing that is becoming quite popular is the karaoke booths. They are private and discreet and only charge around 20$ for the night to sing or shag away. Always the chance of some drunk ass walking into the wrong room and getting more than an ear full of bad singing!
Oh, the life in Japan.
Cheers,
Ashley