Wow, just think of all those poor men, having sex and not knowing that it could lead to fatherhood! Man, we really need to educate these kids....
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I think that men know this pretty well, but as they say, 'A powerful God is the Great God Horn", and wild hormones push this possibility to the very back of their minds.
It is also true that pretty much all men know that there is zero chance of them getting pregnant, and in some cultures and subcultures, knocking up women is concrete evidence to the rest of society that one is a true manly man, hjust as producing offspring is a sign of the womanly woman.
I agree that education for the unknowing is a great idea, as are condoms for the hormone driven.
As Mark Twain commented, it is a lot easier to have responsible sex when one is sixty than when one is twenty. Lust is a vice that affects mankind differently at different ages of life.