I do know women who outright refuse to have sex with men with average penises. Luckily they advertise this need for a giant baby arm ahead of time so most men are scared off. But these girls do exist. Maybe I have the misfortune of continuing to meet them.
Any sex-instruction literature that says that "most women cannot tell the difference between penises of different sizes" was either:
A) written by a man with a very large penis
B) written by a woman who has had only one male sex partner
But as I indicated in my last post (which was only half-sarcastic), there are different uses for small penises versus large ones.