A son takes his father to the doctor.
The doctor gives them the bad news that the father is dying of cancer.
The father tells the son that he has had a good long life and wants to stop at the bar on the way home to celebrate it.
While at the bar the father sees several of his friends and tells them that he is dying of AIDS.
When the friends leave the son asks "Dad, you are dying of cancer. Why did you tell them that you are dying of AIDS?"
The father replies, "I don't want them screwing your mother after I'm gone!"