[Everyone, including men] What sex-related things do you fantasize about, but would never do in real life?

Inspired by a conversation I had with a friend that was about "what things would you consider claiming you like doing or would try, but only in the context of a relationship/romantic partner"?<br><br>tl;dr: If you could do anything you wanted that wouldn't harm anybody (including yourself), but you would not tell anybody about it, what is/are it/are they?