I would've put in some bogus info then said "Hey, I put in some bogus info because I don't think I should *have* to take the survey, but if you make it optional, I'm sure enough people will be willing to help that you'll get valid demographic data."
I'm a reasonable person. I hadn't considered making it optional because quite frankly I was focused most of the day on just making it *functional*. It's a detail that escaped my attention, but one that makes sense since someone else brought it to my attention.