This guy was hitting on this woman really hard for months, trying to get her in the sack. No matter how politely or steadfastly she refused he would keep making racy jokes and stuff.
One day she finally broke down and invited him back to her place. After they were done she said "If I had known your organ was so small I wouldn't have invited you."
He replied, "If I had known I was playing in a cathedral I wouldn't have bothered either"