So, I'm not going to fight it here. I'm just going to allow one authoress, Avarenda, speak about what butt patting means to vampires: the inner torment they suffer at said butt patting.
What I meant by Rosalie 'torturing herself' was that Rosalie (by touching Bella in an intimate manner) is torturing herself by metaphorically getting a taste of the 'forbidden fruit.' Of course, as far as sampling forbidden fruit goes, touching Bella's butt isn't that big of a deal.
HOWEVER, when you factor in the fact that Rosalie WILL NEVER forget what Bella's ass felt like (for as long as she exists) it might be a certain type of agony to recall. Especially when Bella is long gone. (Or even just in the middle of the night when Rose has nothing to distract herself from thinking about it)
I guess (for Rose) we should compare Bella to heroin. Trying it even once will only make her crave for more, than again. Perhaps I'm making a mountain out of a mole hill. I guess we'll see.
Butt patting, it seems then is like heroin for vampires, more tempting, even, than blood. Who would have known?
Rosalie sure didn't.
Until she patted Bella's "very cute" butt.