Four Weddings and a Funeral
The original quote, with slang
Young Bridesmaid: What's bonking?
Scarlett: Well, it's kinda like table tennis, only with slightly smaller balls.
What does it mean?
Young Bridesmaid: What's having sex?
Scarlett: Well, it's kind of like table tennis, only men's testicles are smaller than the balls in table tennis.
"bonking" is British slang
"balls" = testicles