I would say yes it is if you are doing it to communicate that you wish to indulge in an illegal act (sex in a public restroom).
It's the same as if you put your hand in your pocket and use it to make a threatening gesture at someone with your concealed finger that you are holding them up at gunpoint. You can't subsequently claim you never said anything or that you didn't actually have a gun, the intent is clear, and a crime has been committed.
Gosh...I would have thought you actually would have had to proposition someone....like ask for sex or flash what you've got.
OK...Tapping foot = illegal.
Talk about Orwellian.