a substantial fraction of those will not have seen a big enough sample to even guess what "normal" would be

I'm not sure I agree with this. I'm a member of vaginapagina, and women post there all the time concerned about the shape, size, configuration, and color of their genitals, either because they've been mocked and denigrated by partners or because they've seen enough porn to fear that they will be mocked and denigrated. The average person has access to photographs of female genitalia and boys in particular generally see quite a bit of pornography before the age of 18. And then draw conclusions about what is and is not acceptable on a woman's body.