Well, let me tell you: back then I was even less eloquent, even less charming, even less sure of myself with girls than I am now.
Caught your breath yet? Good. Don't laugh so hard, you'll hurt yourself.
I reacted like the proverbial iguana with the proverbial wine. I said 'uh, no', and hopefully added 'thank you'. And they went away. And I was much relieved.
These days, I would /hope/ that I'd have the presence of mind to say something like "I'm sorry, but I have a weird, almost phobic aversion to dancing, but thank you for the invitation".
Ah well. Hindsight.