Quotes from
Four Weddings and a Funeral
Warning: contains bad words

The original quote, with slang   What does it mean?

Young Bridesmaid: What's bonking?

Scarlett: Well, it's kinda like table tennis, only with slightly smaller balls.

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