It's always a question of where you draw the line. Two consenting adults hooking up (whatever you interpret that to mean, I'll assume that the adults are unclothed for simplicity), fine. Being in a car, fine. Hooking up in the private confines of your house, fine. Hooking up in public, not fine. Is a car considered a private location? I guess if someone sees you then no it is not. Hooking up in a camper would be fine. If you put up blinds on the windows of your car does that make it fine? What if you're parked on your own property? What if you're in your garage? What if you are able to find somewhere to park where someone would have to deliberately be trying to see in to see you? There is some case law already pertaining to being naked in your own home. Local laws may be different but are probably similar.