On those facts as listed, it is unlikely someone would be arrested. If any of those facts change, the answer may change.
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I don't see a problem if its unisex.
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Generally speaking, conviction of criminal offense requires "mens rea" -- a culpable state of mind. Anyone can walk into an unlocked but occupied unisex bathroom, intending to use it but startled to discover it is occupied. So where, on these facts, is a culpable state of mind?