Told to me by one of my flatmates.
This young man is invited by his girlfriend to supper to meet her family. In honour of this occasion, she announces to her boyfriend that they can make love for the first time that night. The young man is really very excited about this, it being his first time. He goes to the local chemist and asks the pharmacist there his advice. The pharmacist is very kind and detailed, and for an hour councils the young man on what to do, protection, etc. The pharmacist asks the young man whether he would like to buy a 3 pack, 10 pack, or family pack of condoms. The young man asks for a family pack, as he imagines - it being his first time - that he will be quite busy.
That evening the young man meets his girlfriend's family and sits down for supper, immediately volunteering to say grace. He sits with his head bowed in intense prayer for 5 minutes... 10 minutes... 20 minutes... until his girlfriend nudges him and says "I didn't know you were so religious." To which the young man replies..
"I didn't know your father was a pharmacist."