Your claim, that %80 of people believe this (if proven to be true) would be true.
The claim that my last girlfriend confirmed this to be true (if proven to be true) would also be true.
But the claim alone lacking evidence is not enough to prove the size of my penis.
The person making the claim is the one with the burden of proof. You make the claim, you provide the evidence.
It's that simple. I don't have to disprove your claim, you're claim by definition must first be proven in able for it to be disproved.
It is much like me saying "%70 of the population of the united states thinks marijuana is evil" Does that make it evil?
Then I claim "yesterday crackerboy told me that he thinks everyone on rollitup is a tool, and that he is better than everyone", can you prove me wrong?
Obviously you see the flaw. Just because %70 of people think weed is bad, does not make it bad. Just because you can't prove you didn't tell me yesterday you think everyone on rollitup is a tool, and that you are better than everyone, doesn't mean it's true..
But if you want to look at it logically:
The chance of me having a small penis is slight, say %10
The chance of me having a large penis is slight, say %10
The chance of me having no penis, is very small, say %1
That leaves %79 chance that my penis is within one or two standard deviations of the average penis size.
Unless there is evidence as to the size of my penis, any rational intelligent person would know that chances are I have an average sized penis.