One Sunday morning Joe burst into the living room

One Sunday morning Joe burst into the living room and said, "Dad, Mom, I have
some great news for you! I am getting married to the most beautiful girl in
town. She lives a block away and her name is Susan.
After dinner, Joe's dad took him aside, "Son, I have to talk with you. Your
mother and I have been married 30 years. She's a wonderful wife but she has
never offered much excitement in the bedroom, so I used to fool around with
women a lot. Susan is actually your half-sister, and I'm afraid you can't marry
her."
Joe was heart-broken. After eight months he eventually started dating girls
again. A year later he came home and very proudly announced, "Dianne said yes!
We are getting married in June."
Again his father insisted on another private conversation and broke the sad
news. "Diane is your half-sister too, Joe, I am very sorry about this."
Joe was furious! He finally decided to go to his mother with the news.
"Dad has done so much harm. I guess I am never going to get married", he
complained. "Every time I fall in love, Dad tells me the girl is my
half-sister."
His mother just shook her head. "Don't pay any attention to what he says,
dear. He's not really your father."