I would say it's rated so lowly because Hoopa Unbound, the "girl" getting fucked in this, is basically just a big muscular man with 6 arms and a beard, who just also happens to have a vagina. It spends so long building up to the fucking and then the actual sex scenes aren't pleasing at all to look at, it's way more than just disappointing. If they had changed Hoopa's design to look a lot more feminine, or even just scrap Palkia fucking Hoopa and have May jerking Palkia off through the portal or something similar, it would've been much better.
Base +6, kicking -6, Garfilth +4, Alagarazu +8, no-name here +7, kemono69 +12, Anonymous mystery +7