No Horcrux AU, NoWar (aka PostWar). Finally was able to figure out how to prompt this one out without going super dark.
In an effort to get under Draco's skin, Harry goes to convince Tonks to help him with a prank. Tonks, always interested for a good lark, agrees to disguise herself as Narcissa who will hang off of Harry and mewl at him while they walk through Hogsmeade. Harry eventually gets past the hot women on his arm bit and possessively paws at Narcissa!Tonks. Suggestive hand placements and the tittering giggles makes it "obvious" how close they are to all on lookers.
It all comes to a head a few weeks later when the rumor mill has led to the largest divorce in Wizarding Britain's history. With a recently single Narcissa who is out to hurt Lucius as much as possible, Sirius affronted that he wasn't included in the prank and aiming to "one up" the Harry and Tonks' prank, and an out of the loop Harry unaware of the ramifications of his actions, we get news hitting the Daily Prophet of a "marriage contract" between Harry Potter and a "daughter of House Black".
See, Sirius wanted to teach a lesson to Harry and Tonks (and push them together). Instead we have a public response from Narcissa about her acceptance and a push that the contract be followed through. Now the public is invested and is like a dog with a bone and won't let it go.