After the war, Harry has amassed a bunch of titles. Which sounds great on paper since more assets > fewer assets and there's a delicious irony in donating the fortunes of Death Eaters to various charities.
However, there is one issue: after his engagement to Fleur, he is informed that he needs a wife for every house. Which is decidedly less fun if you are already in a committed relationship. (And given how Fleur wears him out, he is unsure if he'd be of any use to additional wives). And if he does not, all properties of the families in question will revert back to the Ministry, which is proving to be remarkably corrupt and resistant to reform even after the war.
The solution comes to him in a pub. They were showing a boxing match on the telly and after that ended and he was already a few pints into the evening, when he had a sudden idea. He needs a wife for each title, but there is no regulation that the wives have to be different people.
So he does exactly that and has 7 different marriage ceremonies, all with Fleur. And to throw the British establishment even further off track, he gets the French ministry to bestow a bunch of empty titles on Fleur, so the marriage announcements each read differently (Harry Potter for House X and the Lady of (insert random French title)). And funnily enough, no one notices at first. Harry and Fleur even start to fake appearances (Fleur is disguise) to see how long they can mess with the pureblood establishment and enjoy the wild speculations each "new" wife causes.