She recommends a more efficient approach

Fleur proposes a suggestion which makes Ron’s freckles bleach white, “If you ahr in such a rush, Ronald, we don’t have to take turns. Come, join me!” She widens her seated stance and pats the toilet seat directly in front of her pussy. “There’s plenty of room, don’t be shy!”

Ron, of course, is mortified by the Veela’s indecency, but recognizes that to the French girl, he must appear to be the unusual one. He gingerly drops his pants and steps forward hesitantly. Apparently unsatisfied with his pace, Fleur grabs his hips and plants him firmly on her lap. She deftly stuffs his penis down to point into the toilet, making Ron whimper at her guiding touch, and she gives Ron a beautiful smile. “Go on, no need to wait.”

Ron worries that piss isn’t the only thing he’ll expel, but he obeys as best as he can. He clenches his abdomen, and starts to leak urine into the toilet bowl.