Anywhere that allows users to speak to one another this seems to be the case. Across culture and now decades of internet (and before) human nature is about the same.

After meeting a few people on FetLife first, pretty wild. Of course some under delivered or weren’t a good match but generally it’s an online location for kinky weirdos.

By your definition of secret nearly everyone I’ve ever slept with was in secret, although I’m not a good sample. I’m essentially an orphan and I have no “friend group” only a collection of weirdos I hang out with individually. Aka everyone in my life is a secret person by your definition.