uhh SPOILERS
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First of all Heesung is very tense and scared that Chiyoung will turn into a wolf again and hurt him leading up to the sex and even during, like I felt so bad for him.
Even the sex, Heesung still wasn't ready but since he was scared ML would go into pheromone shock again (after finding him in the dog gang fight) and because he feels guilty that he's going into pheromone shock because of him, he kind of lets him but doesn't think they'll go all the was through.
It's not enough for Chiyoung and they have a rapey, kind of violent sex, where Chiyoung doesn't care abt Heesung at all.
They show Heesung 'feeling good' but idk the whole scene was icky to read, and with the way the story has been so fluffy and romantic until now I was expecting their first time to be romantic and gentle ://
someone linked the novel in the comments and man their first sex scene is so disappointing *spoiler* I let myself relax with all the fluff just to have it proven once again that Korean bl authors cannot stop writing noncon and borderline rape