[D]o you know the reason *why* the Halacha is that men are prohibited from walking behind women? The same reason could easily be applicable to sitting behind them, especially for an extended period of time.
The reason, according to Rabbi P. E. Falk (in Oz v’Hadar L’vusha) is “so that he is not
drawn to watching her as she walks.” That’s obviously not applicable when she’s sitting down.
He is in favor of not having women be where they are visible to men, though,
based on Bereishis Rabba about Eliezer riding ahead of Rivka and her servants.