threshold, women's quarters
閫 centers on a physical or social boundary: a threshold that marks an entrance, and by extension the inner women's quarters separated by such a boundary.
閫 combines 門 (gate) with 困, which likely contributes the sound and the idea of being enclosed or confined, together suggesting a threshold or inner quarters.
A gate 門 encloses a confined space 困, like a threshold separating the outside from the private women's quarters within.
For コン, imagine a con artist trying to cross the threshold into the women's quarters: con -> コン, and the gate 門 stops him.