I usually reply by asking if I can take their woman "out for a spin"!
Seriously, I HATE when people ask to ride your bike! To me, it's incredibly rude to even ask. Your bike is a very personaly thing, and the potential for someone doing damage to it, even unintentionally, is too great.
If you offer
to let someone ride your bike, that's one thing. I still don't recommend it, unless it's someone you REALLY trust, but at least you
are the one who brought it up.
But having someone ask you is just plain awkward. It's like someone asking, "Hey, can I borrow $1000 for just a couple of days?", or "Hey, I need a place to stay for a few weeks, can I stay at your place?"
I mean, what are you supposed to say, if it's a friend? You want
to help out, but on the other hand, you're thinking -- F*** No!
A fool and his money are soon partying.