Ore No Osananajimi No Seibetsu Ga Wakaranai - Oneshot

[HaxtonFale]

It reminds me of another manga where the MC promises an illegal marriage. At the end of the manga he becomes a politician and changes the law, just so he can fulfil his promise.

There was a bit in Fujimura-kun Mates! where Himuro, who has skipped grades despite it not being a thing in Japan, eventually became the PM and legalised polygamy so all the girls after Fujimura could marry him.