A man and a woman get married, and when she moves her things into his house he tells her that there is a box in his closet, that if she ever opens it that he will instantly divorce her, no if's and's or but's about it. She thinks about it but agrees, and they are happily married for 50 years.
Shortly after their 50th, the man falls ill and is on his deathbed in the hospital. The wife was visiting him then went home to rest and change before going back. She opens the closet to get a change of clothes and sees the box and thinks to herself, well, he's not going to be coming home so what's the harm in peeking.
Inside the box are 2 golf balls and $1500.
Later on when she's back at the hospital, she admits to opening the box and tells him she's confused as to the contents. He tells her that for every time he was unfaithful he placed a golf ball in there. The wife is disappointed but thinks twice in 50 years isn't that bad.
Then she asks him about the $1500, to which he answers "Well, every time I got a dozen golf balls I'd sell them!"