Question: Is tapping your foot really code for public sex?
Yes. The signal has been around for decades in the United States and Europe. Generally, one person initiates contact by tapping his foot in a way that's visible beneath the stall divider. If the second person responds with a similar tap, the initiator moves his foot closer to the other person's stall. If the other person makes a similar move, the first will inch closer yet again. The pair usually goes through the whole process a few times, just to confirm that the signals aren't an accident -- Christopher Beam/Slate.
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Related: Same sex: Larry Craig's anti-gay hypocrisy/William Saletan, Slate
DFO: This is probably more information than you wanted to know.