"My fiancee, Ran-chan." She felt anger and pain rising within her. "He
promised long ago to look after me forever. And now he's... he's married
another woman!"

-------

Did Ranma ever promise this?  Or is this Ukyo's imagination at work?
I suppose you're assuming something not explicitely stated in the manga.

{Ranma made just that promise, revealed in the Secret Sauce storyline.  
Note however, that a promise to take care of someone does not 
necessarily constitute an avowal of intent to marry.}