I will disregard my own strong distaste for using such a thing as rape as a plot 
device for the time being to make the following comment.

Rape puts the aggressor at an extreme disadvantage, combat-wise.  Unless the 
victim is first beaten nearly unconcious or either restrained or drugged in some 
fashion I don't see it happening.  That is assuming of course the victim chooses 
to fight back, which I would definitely assume Ukyou would be doing.  Even if 
the man can succeed in something along the lines of a wrestling "pin", this does 
not prevent Ukyou from say, headbutting or biting.  Or using her legs (jujitsu 
anyone?).  

Unless the man who raped Ukyou is along the same power level as say, Nakago or 
Darth Vader, I don't see how he could of successfully raped her.  Given that she 
is a) excessively strong and b) not the kind of person that would stop fighting 
back.

Why would Ukyou need Ranma to defend her?  I think she is a stronger person than 
that...  

-Ben.

A short note:  I have taken part in and helped others practice "IMPACT", a 
woman's self defense course.  I disagree with some (many, actually) of the 
things that they teach, but personally believe that it is quite a bit better 
than nothing.