Louise is the type of girl who'd only enjoy sex being subdued and humiliated, and I firmly believe she'd only reach orgasm being taken by force. My theory is that she treats Saito like trash because each time she does that she hopes he'd finally pop, take her whip and put her in her place.
He then would proceed to tear her clothes, put her in all fours and mount that virgin ass of hers while pulling her pink hair with one hand, spanking the butt with the other and yelling "WHO'S YOUR DOG? WHO'S YOUR DOG NOW, BITCH?"
Believe me, I know the type.