I agree 'public place' can definitely throw up a lot of grey areas.
But the common factor is that the penis is being exposed in an unsolicited way to a non-consenting woman.
Surely this is the essence?
We used to say that if a person with a penis used that penis to penetrate a woman with whom the penis person had signed a marriage certificate that it was not rape. We now agree it is rape (I assume).
So, even if an individual with a penis exposes said penis to a woman in the context of marriage when it is unsolicited and unwanted then surely it is flashing?
And with that as the bar, surely the plumber exposing a penis is flashing?