Sex or chocolate

A missionary gets sent into deepest darkest Africa and goes to live 
with a tribe therein.  He spends years with the people, teaching them 
to read, write and the good Christian ways of the white man. One thing 
he particularly stresses is the evils of sexual sin. Thou must not 
commit adultery or fornication!!

One day the wife of one of the Tribe's noblemen gives birth to a white
child. The village is shocked and the chief is sent by his people to
talk with the  missionary. "You have taught us of the evils of sexual
sin, yet here a black woman gives birth to a white child.   You are 
the only white man that has ever set foot in our village. It doesn't 
take a genius to work out what has been going on!"

The missionary replies: "No, no, my good man. You are mistaken. What 
you have here is a natural occurance - what is called an albino. Look 
to thy yonder field. See a field of white sheep, and yet amongst them 
is one black one. Nature does this on occasion." The chief pauses for 
a moment then says "Tell you what, you don't say anything about the 
sheep, I won't say anything about the white child"