Assuming all other traits are equally spread out among penis size, for instance looks, confidence and charm, women are only going to encounter an 8"er once, on average, if they've had sex with 50 men.
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Unfortunately Murphy's Law might state that no matter how big our penis is it's a probability that someone else's is bigger. Then again, according to Kinsey it would be highly unlikely you will be with a girl that has had one bigger than 8x6. But what if she did? I hardly think she'd be anything less than THOROUGHLY IMPRESSED if your little 8.5x6.5 was ONLY the second biggest she's ever seen.