One day a nun got into a cab.
The cab driver, noticing she was a nun, and asked her if she would ever have sex. After she had thought about it for a while she said "Well, yeah I would have sex with a man if he had never been married, did not have any kids and went to church every Sunday."
So the cab driver said "What do you know, I don't have any kids, never been married and I go to church every Sunday!" So he asked her if she would have sex with him and she agreed as long as he would take her from behind.
So they had sex and afterward the cab driver said "Ha ha, I have six kids, I've been married three times and I've never been to church a day in my life!!"
And the nun pulled off her mask and said "Ha ha, I'm a man going to a costume party!"