Just drop a note to the manager or owner and tell him you're no longer a customer and explain why. Give names or descriptions if you can. Otherwise it's not worth worrying about: Bike shop employees, at least around here, are notorious *****holes. One local shop owner changed his approach a couple of years ago and began advertising as "your family bike shop," selling comfort bikes and cruisers and whatever people needed in addition to his high-end line, and his business was up 30 percent the first year.
For the record, though, I think you're being too sensitive when you consider the question about the 'bent to be an insult. It was a perfect time to say something like, "Yeah. I rode a regular bike when i was your age, but by the time I was 25, I was pretty much impotent. It took two operations and some therapy to get things working again, and now I stick with the recumbent. My doctor said he sees that in young cyclists all the time..."