I was thinking of making a more detailed version of that statue in the Purple Room the NPC, and have it be some Lust deity that has chosen [the player] as their herald to use his emotional connection to and relationship with the girls to turn them into bimbos and increase her influence and ability to manifest. Simply by thinking about them and speaking their name in her presence.
Mostly just got this idea from going by the "broken hearts make the transformation stronger" comment From ole Todd.
This also leaves in the options for multiple bimbo magic producers as simply being other chosen heralds.