Hmm. The big cock idea may not be quite what it seems. You might want to look into the "human penis as semen displacement device" theory. The idea is that a male does not know whether the female he is mating with has had sex with someone else - so the shape of the penis is designed, sure, to deposit semen in the vagina close to the cervix, but also to displace any other man's semen.
Have a look at this attachment.
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