If she says you're not having sex tonight, you're having sex tonight.

This seems to be general wisdom. What exactly does it mean?

The best I can come up with is:

If she feels the need to say you're not having sex tonight, it means that:

  1. She is thinking about sex
  2. She knows that sex is a possibility, otherwise she wouldn't feel the need to warn you otherwise

Is that the meaning, or is there something else behind that phrase?