Don't know if you've heard it but....
A little boy and his father are walking down the road when they come upon 2 dogs having sex. The boy asks his father what they are doing. "Well", the father says, "they are making a puppy".  Later on that night the little boy walks into his parent's room and they are making love. The little boy says, "what are you doing?"  "Well", the dad says, "we are making you a little brother or sister". The little boy promptly replies "Well, flip her over dad I would rather have a puppy".