Okusama ga Seito Kaichou!

The story begins with Izumi Hayato running to be student body president. but when a beautiful girl swings in promising the liberalization of love while flinging condoms into the audience, he ends up losing to her and becoming the vice president. At the student council meeting, the newly elected president invites herself over to Izumis house where she promptly announces she is Izumi's wife through an agreement between both parents (and some liqueur) when they were only 3.
Includes: Chapter 20.5