Ramsay is a sadist of the highest order.
Sansa is only a step above Theon/Reek in the spineless category.

This ep is the 1st time she ever showed heart. (Putting ole girl in her place) Way back when, she sold out her sister at the prospect of being Joffrey's Queen. The only thing I can see waking her up, would be protecting her child....hmmmmm

Theon has always been a f--k up, and now that he is fully broken, whenever he decides to rebel, he gonna mess that up too.

But all in all that rape definitely fits the story/characters involved.