XYZ Affair

A middle-aged couple who had two stunningly beautiful teenaged daughters. They decided to try one last time for the son they always wanted. After months of trying, the wife became pregnant and sure enough, nine months later delivered healthy baby boy. The joyful father rushed to the nursery to see his new son. He took one look and was horrified to see the ugliest child he had ever seen. He went to his wife and said that there was no way that he could be the father of that child.
"Look at the two beautiful daughters I fathered." Then he gave her a stern look and asked, "Have you been fooling around on me?" The wife just smiled sweetly and said, "Not this time."