Responses
I think the problem lies in how they narrate it and the fact that Ginga felt guilty about it, when all three of them are in love in the end and Yuzeen enjoys watching them play together, so when they did behind Yuuzen's back, it felt like cheating because they believe what they are doing is wrong
So when the doctor said they shared a certain similarity in that both alphas shared fluids, she was referring to what happened in the extra and why they both got nervous when the doctor mentioned it. Yuzen didn't seem surprised because he realized how perverted his partners are, but honestly, I would have liked them to include it in the main story and tell Yuzen about it, since it felt like cheating. And even though the extra was good, I didn't like the ending. I hope the sequel mentions it because it seemed to me that Ginga suffered for nothing in the end. I would have loved it if they had swapped positions; after all, Ginga already did it with Kou.