
why is no one telling him(╯°Д °)╯╧╧

Because he's a female lycan who believes he is a male lycan (his memories got altered)
His subconscious may know he is female (when he talks to his other self: the wolf) but he lives his life as a male
Dropping this bomb will cause him stress and his body will reject the triplets. Hwa is trying to get his memory back so he can get Seol's true memories back as well.

In the story, the uke doesn't know that he's a female lycan because someone has reset his memories in exchange to not erased his memories about Hwa, they've been acquintance since kid, and seolhyun already fall for Hwa long time ago, but because Uke is the pure breed female lycan, their clan forbid him to have relationship with half-vampire-lycan Hwa, (maybe they kinda traumatic because of Hwa gok whom the previous female lycan end up with Yibin who is a vampire)
Listen, I am about to play devil's advocate here, how is he supposed to notice a small tiny ring in a place you were not even meant to be????(╯°Д °)╯╧╧
Maybe the ring have gps with it. Idk lol let's wait for yaoi logic