It's culturally expected for women to use diapers

When indoor plumbing became common in this culture, women weren't interested in it. The cold toilet seat was unpleasant, going into a bowl of water seemed distasteful, the whole process felt degrading and unnecessary. Most women preferred to wear diapers, and indeed, the idea of a woman in this culture using a porcelain toilet feels as perverted and wrong to most people as a grown woman wearing a diaper would feel in our world.

Diapers are somewhat fetishized, as anything worn in such an intimate place would be, and there are various "sexy" diapers available and most women like wearing an attractive diaper. However, they're still fundamentally a utilitarian thing to women in this culture. Women's bathrooms still exist but they have no toilets, and are instead places to change diapers if needed.