There's a group of people, men and women, who take this beyond kink and consciously try to "correct" women's behavior with corporal punishment. My guess is the girlfriend is into spanking and stumbled into this culture online, and seeing it as a way to fulfill her kink suggested it to her boyfriend. But that's a (fucked up) lifestyle that isn't just about sex, it's about treating women like abused children. I can see an appeal, it excuses her from any responsibility since it assumes she cannot make decisions without 'training', but she's probably struggling with the whole concept. Hopefully they can work together to figure out how to meet her kink desires separately from the work she needs to do to live a responsible adult life.