Son: "Dad, I fell in love and want to date this awesome girl."
Dad: "That's great son! Who is she?"
Son: "It's Sandra, the girl next door."
Dad: "Aah, I wish you hadn't said that. I have to tell you something son, but you must promise not to tell your mother. Sandra is actually your sister."
The boy is naturally bummed out over this revelation from his Dad, but a couple of months later...
Son: "Dad. I fell in love again, and this one is even hotter!"
Dad: "That's great son. Who is she?"
Son: "It's Angela, Sandra's sister."
Dad: "Aah, I wish you hadn't said that. Angela is also your sister."
This went on a few more times, and finally the son was so mad he went straight to his mother.
Son: "Mom, I am so mad at Dad! I fell in love with five different girls and I can't date any of them because Dad is their father."
Mom: "You can date whoever you want. He isn't your father!"