"Mating only with the guardians"
I noticed that too. What's up with that? (I mean, other than a Japanese protag trying to avoid having sex (despite the setting...))
Hm, both girls he mated with who didn't get chokers were masturbators. Could it be he needs to break the hymen himself to make the choker? EDIT: On hindsight that can't be it since Pitora (or w/e her name is) shouldn't have hers.