I don't see it. It's a tree that people have sex on. Gay sex, though from the looks of it the tree would be equally well suited for lesbian or straight sex. Presumably one person lies on their stomach on the trunk while one or more people perform penetrative acts. Where is the euphemism? And what is weird about listing this on wikipedia?
I wonder if it has been ruined by becoming well known.
It is sort of funny—based on the Wikipedia they put up notices to stop people from having sex there, which didn’t work. They should have advertised it instead, maybe, the surest way of ruining something niche is to let everybody know about it.
[1] https://www.theartnewspaper.com/2023/12/07/hampstead-heaths-...