We are Pagans, and have often had sex at sacred sites. We have kissed on the penis of the Long Man of Wilmington; couldn't do more as it was broad daylight. Couples used to fuck on it in times past to help conceive children, some still do we understand.
To find an ancient sacred site near you, google the Megalithic Portal or the Modern Antiquarian websites. Beware that the larger places are more likely to have visitors, or even security, like Stonehenge. There are lots of megaliths, stone circles, sacred wells, long barrows or round barrows around, usually isolated.
Be respectful to the spirit of the place, don't light fires or leave rubbish. Many of these places were used for fertility rites and sex will almost certainly have happened in ages past. Beltane, 1st May, was traditionally the night young couples went into the woods for sex. If girls fell pregnant and didn't marry, their children took the surname "Greenwood."
You might not be the only ones there in warmer weather, pagans often hold ceremonies on full moon nights and at tyhe solstices, equinoxes and the ancient fire festivals in between these events.
There's an old joke -
Q: How many Druids does it take to screw in a lightbulb?
A: Druids don't screw in lightbulbs, they screw in stone circles! 