The Vicar's daughter is about to marry a sailor. On the night before the wedding, the mother says to the daughter "I know what these sailors are like. He may ask you if he can do it 'the other way'. It's dirty and it's sinful, so say no."
They marry. A few months in to the wedding, it is the sailor's birthday. The wife is feeling a bit sinful and dirty, so she says to her husband, with a wink and a grin, "As it's your birthday, do you want to do it 'the other way'?"
"No thanks" he replies, You might get pregnant"