As tro doing things (illegal) in the dark:

Most parks close at dusk, because it is not easy to uphold the law in such places in the dark. Parks attract people who do things in besides slide on the playground equipment (i.e. use drugs).

It may not be illegal to "make out" in your car after dark? (However, it may constitute a lewd act if you make out in public without a partner, aka George Michael). But, it may be be illegal to be at a park after a certain time of night?

Beside that, it's probably not good for the car upholstery.