Why can't I date?

I kid wen't to his father and asked,
"Daddy, I fell in love and wan't to date this nice girl"

Father: "That's great son. Who is it?"

Son: "It's Sandra, the neighbor's daughter."

Father: "Oh, I wish you hadn't said that. I have to tell you something son, but you must promise me not to tell your mom Sandra is actually your sister."

The boy is naturally bummed out, but life goes on, a couple of months later ...

Son: "Daddy, I fell in love again and she is even hotter!"

Father: "That's great son. Who is it?"

Son: "It's Angela, The other neighbor's daughter."

Father: "Oh, I wish you hadn't said that. Angela is also your" sister.

This went on couple of times and the son was so mad, he went straight to his mother crying.

"Mom I am so mad at dad! I fell in love with six girls but I can't date any of them because dad is their father!"

The mother hugs him affectionately and says:
"My love, you can date whomever you want. He isn't your father"