Usually it's the guy who ends up building his own harem, but here it's MC's girlfriend who's building a harem for him instead. That's seriously rare
....and the one who legitimately seems most excited about this is the GF.Imagine trying to live a normal peaceful life with your girl and then she tells you that she wants you to have a harem, already has a set of rules ready, and also already has several girls lined up, including your neighbor and a celebrity.