Only, afterwords (several chapter later), it turns out the ritual is actually a marriage ceremony, so some of the girls - who are betrothed to guys they like - back out so they can marry their men. The rest don't like their husbands-to-be. (Harry eventually wins over the Patil Twins dad by saying he would support his bid for lordship?)
Also, this part might be from that harem fic, or from a different harem fic. But, Harry builds a magical building in the Grangers back yard (I think, I'm sure), and it has a giant underground wizardspace room built by goblins that has mountains, rivers, forests, beaches, etc in it.
It is, also, quite likely to be on fanfiction.net