The two countries had an agreement that all spies had to be returned alive and with no signs of physical harm. Even a single bruise could be considered a declaration of war.
It was always so easy to get answers out of male spies. A few hits to the balls and they would spill all their secrets.
But these female spies were impossible to crack. Without balls, they had no weakness to exploit. And they couldn’t use any worse torture without leaving physical signs.
They were so superior to male spies, the countries decided to upgrade all their spies to be women. Why use the inferior model when a better one is available?