>>>>>>>>>>>>>>>
He saw Ranma-chan naked sitting on a stool in the men's bath, so how
come there was no nose bleeding
 or even a little bit of embarassment? 
 1.  Maybe Ryouga doesn't consider Ranma-chan a real girl?
Hence no real embarassment.
I quite like this story but it seems odd that he does not have his usual
reaction to seeing naked girl, even from behind.
<<<<<<<<<<<<<<<

I think this option is the most likely here.  Wait until you get to tankouban
#16, when Ryoga sees Shampoo naked.  Totally loses it...  ( I hope that's
not too much of a spoiler...)