It was a single swat to the buttocks, done in full sight of everyone. There was other ass-spanking going on, between people who knew each other - done as a joke - so in context it was not so unusual. I would not have done it outside of that context, nor would I have done it if my inhibitions had not been lowered by alcohol; nor would I do it again even if they are.
Yes, she deserved it!
It was a mistake. Why? It exposed me to more risk than was worthwhile, and while I might have hoped that (aside from simple punishment) it would teach her the lesson that she ought to follow the Golden Rule, or at least should not pull the same tricks on guys, in retrospect it was unlikely to do so.
Other people (that I have talked to) seem to be divided on whether it was a good thing to do or not.
I think this situation falls pretty squarely into "two wrongs don't make a right" territory. The moral intuition is that a minor social infraction doesn't justify a violent response, even extremely minor violence. Even though you don't say so, perhaps that was a tacit reason for you to acknowledge it as a mistake.
I do sympathize with your frustration at encountering such naked privilege and entitlement on her part, and that you would want some sort of recourse. It's possible that such brattiness would cause her trouble in her future relationship...
Followup to: Do you have High-Functioning Asperger's Syndrome?
LW reader Madbadger uses the metaphor of a GPU and a CPU in a desktop system to think about people with Asperger's Syndrome: general intelligence is like a CPU, being universal but only mediocre at any particular task, whereas the "social coprocessor" brainware in a Neurotypical brain is like a GPU: highly specialized but great at what it does. Neurotypical people are like computers with measly Pentium IV processors, but expensive Radeon HD 4890 GPUs. A High-functioning AS person is an Intel Core i7 Extreme Edition - with on-board graphics!
This analogy also covers the spectrum view of social/empathic abilities, you can think about having a weaker social coprocessor than average if you have some of the tendencies of AS but not others. You can even think of your score on the AQ Test as being like the Tom's Hardware Rating of your Coprocessor. (Lower numbers are better!).
If you lack that powerful social coprocessor, what can you do? Well, you'll have to run your social interactions "in software", i.e. explicitly reason through the complex human social game that most people play without ever really understanding. There are several tricks that a High-functioning AS person can use in this situation: