The basic idea here, of Akane's rape being the trauma that shaped her 
personality, is one with some potential but...

She just up and tells him.  

He doesn't even have to work for it.  

It doesn't really arise out of the situation.  They're alone together, 
and, as usual, Ranma isn't doing anything, and isn't likely to do 
anything.  

Akane doesn't have to tell him...  The truth is, Ranma's too clueless to 
notice she's not a physical virgin, and given her extensive martial 
training, the chance that she'd be virgo intacta is insignificant 
anyway.  

Ranma and Akane aren't exactly poster children for sharing their 
emotions anyway.  Even if they were, this shouldn't come totally out of 
the blue this way, for dramatic reasons.  

My suggestion?  Start out with a nightmare, or Ranma and Akane starting
to get romantic, then she suddenly turns cold when he starts to take the 
initiative.  At that point he can either start asking questions, or in 
the latter case perhaps start saying that maybe there's no future in 
marriage for them if she isn't physically interested in him (perhaps 
assuming that it's the curse).  Give her a reason to open up.