Spanking is inherently violent. It is also inherently sexual.
Study after study tells us that this kind of bullying in the name of discipline is leading to the high level of violence and child maltreatment experienced in this country. The thought of an adult, supposedly a caring adult, using such an instrument to "discipline" a child is beyond comprehension.
And despite tons of evidence that spanking is bad for kids, lots of Americans seem to be OK with it.
Spanking can be used as a form of foreplay or it can be a stand-alone event. I have found that spanking during coitus can bring on the most powerful orgasms, I am talking serious full-body-holy-crap-orgasms that you will be talking about for weeks.
Being held in just the way you need to be by someone stronger than you are in that moment, being told what to do in the safe context of a mutually agreed upon power exchange, is pretty damn intoxicating. That is what Christian was trying to offer Ana.