Testing the Harm Principle

So popular is the woodsy field below the ridge as a spot for gay sex (mostly during the day) and heterosexual sex (mostly at night) that the police have designated it a “public sex environment.”
Public sex is a popular — and quasi-legal — activity in Britain, according to the authorities and to the large number of Web sites that promote it. (It is treated as a crime only if someone witnesses it, is offended and is willing to make a formal complaint.) And the police tend to tread lightly in public sex environments, in part because of the bitter legacy of the time when gay sex was illegal and closeted men having anonymous sex in places like public bathrooms were routinely arrested and humiliated.

I wonder how this interacts with England's many footpaths (legal rights of way, dating back many centuries, that often take one through private land).