Hmpf. Many of the people I've known over the years have revealed stuff about themselves that I'd much rather not have known.
If a particular practice is socially frowned on by some, then the substantial minority -- or sometimes even a majority -- that engages in it may hide its behavior, leading everyone to dramatically underestimate the prevalence of the practice...
The quote ["Growing up in Seattle, I knew nobody who owned a gun"] reminds me of Justice Powell's famous line "I don't believe I've ever met a homosexual," said at the time Justice Powell was considering his vote in the Bowers v. Hardwick (1986) gay sex case. (See John C. Jeffries, Jr.'s biography.) Powell had by then had several gay clerks, and apparently said the statement to a clerk who was himself in fact gay. Powell's belief that he'd never met a homosexual was much like some people's belief that they didn't know anyone who owned a gun.
This also helps show the wisdom of many gay rights activists' view that coming out to friends and family is itself a potent political action. It's much harder to demonize that which your friends happily do than that which no-one you know would ever dream of doing.