Someone is organizing the system to pay the lowest possible wages. This someone operates within deeply entrenched structures of power. Someone also didn't let women vote until 1920 in the US... 1959 in Switzerland. And someone made sure that inheritance customs were patrilineal... someone admired extremely patriarchal societies like the old Roman and Greek... someone treated women as property... someone kept women out of the universities... someone kept them out of the guilds and professions... who is this guy?
That's the long story of the struggle for justice for women.
You seemed to say that gender injustice could only be perpetrated by some shadowy caricature "someone." I'm saying there is such a someone: the patriarchy.
On an individual level, perhaps, but how would individual negotiations explain the difference in pay between whole professions (where most of the pay gap is)?