Mousse did it,

Mousse is a bit of an outsider to the Amazon village so his mind works differently. He understands that Japan is a male dominated society. He hates Ranma for dragging Shampoo down to such a demeaning level, unfit for an Amazon warrior maiden and becomes even more enraged when Ranma does show some affection.

Not able to take it anymore, Mousse figures Ranma’s life is dictated by honor and devises a plan to trap the enemy of all women as a Japanese house wife, while trading his duck curse for a female curse to become a high ranking member of the Amazon tribe. Since the Amazons protect pregnant females, Ranma is no longer available groom material. Cologne and Shampoo can thus release Ranma without marrying or killing him. Wanting nothing better than return to China with Shampoo’s honor restored, Cologne decidedly overlooks Mousse’s use of her potions.

John Ma