a sexual practice somewhat like penis fencing

Is that anything like One-legged Herring Duelling?

One-legged Herring Duelling

This requires two idiots, two herring, and stout washable rainwear. It should be played outdoors. Contestants stand on one leg facing each other, holding their right foot in their left hand for balance. In their right hand, they each hold a herring by one end. They hit each other about the body with the floppy ends of their herring for one minute; the winner is anybody who refuses to play, closely followed by the player whose herring is most severely damaged by the experience.