I've never been to Paris, and all the websites I've looked at are very much focused on tourist attractions. I looked at some photos of the Bosques, but they seem terribly remote, especially back then.
Judging from photos, I don't see clandestine sex being very possible in any of the formally landscaped parks, like the Garden of the Tuileries, for example. Unfortunately I'm at a loss for other options. Any ideas?