I'll be optimistic and assume the latter option happens! I've heard this reply several times, here's what happens next:
Alicorn: Hey, wanna buy me a drink?
cousin_it: (smiles happily and shakes head)
Alicorn: Aw, why not?
cousin_it: (keeps smiling, almost laughing, eyes half closed, reaches with arm to catch her waist)
In fairness, you can't give an informed reply to that because you can't assess my physical attractiveness over the Internet, but I can just tell you the decision tree from this point. The girl either plays along or evades. If she plays along, I keep doing what makes sense. If she evades, I turn away to the bar without lingering even a second. You'd be surprised how many girls thus NEXT'ed later come back :-) Of course I don't mean to imply anything about your behavior!
Yeah, going for the waist at that point would get a shriek even if you managed not to tickle me, and not in a good way. I don't care if you look like Sean Maher. I'd escape (and it would feel like escaping, not like something more neutral like "disengaging" or whatever), and if I was with any female friends I'd warn them you were grabby. I might do an evaluation of how the bouncer would react if informed, but I have low priors on getting help for "socially acceptable" invasions of space.
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: