A sailor comes to port, and decides he really needs to get laid. So he walks into town, and sees a sign: “Sisters of Mercy Convent & Brothel”.
He walks into what appears to be a church. There’s a nun sitting by a table near the door. She intuits what it is he’s looking for, and says, put $5 in this tin cup and walk through that door.
The sailor does so, and enters another room, and there’s another nun, with a table and a cup. $10, she says, and go through that door.
The sailor goes into a third room, where a third nun with a table and cup asks for $15, and go through that door. The sailor coughs up the dough, and walks through the door.
It slams and locks behind him, and he is in an alley behind the church, facing a brick wall with a large sign: “YOU HAVE JUST BEEN SCREWED BY THE SISTERS OF MERCY!”.