"Simple, just come down to the river tomorrow and we'll show you."
The next day the doctor shows up and sees a group of Indians near a donkey.
One man says "Since you're our guest you get to go first."
The doctor not wanting to go against custom starts to kiss, then proceeds to have sex with the donkey.
Then one of the braves in the group asks "Are you almost done Doc?"
"We need the donkey to cross the river in order to get to our women."