After one too many annoying gossip articles in the Daily Prophet, Harry is boiling with anger. The Weasley twins suggest to prank the prophet and bait them into writing some twaddle which is so obviously false that they will look like a fool for ever believing the story. So after a short brainstorming session, Harry approaches Fleur about pretending to be an item, which would have the added benefit of stopping those love triangle rumours about Hermione, Viktor and him. So the next Hogsmeade weekend, they go on a date (Harry asked Lavender for a few pointers about the most romantic things you can do at Hogsmeade), and let's just say the ruse works too well...