OK let's try this. I've faced this situation maybe hundreds of times, and am curious as to how you compare to the girls I typically see.
Alicorn: Hey, wanna buy me a drink?
cousin_it: (smiles happily and shakes head)
Your response?
Then it would probably depend on how much I wanted to talk to you in the first place (where this includes, factored in, how charming the happy smile is). If not much, I'd probably shrug and go back whence I came. If more than not much, I might say "Aw, why not?" - not to try particularly to extract a drink after all, but out of curiosity and to have something to have a conversation about.
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: