Yeah, it kinda ruin it for me too. But considering that sensei (the author) made these extras means that many fans might've requested for Kou x Ginga. In which in didn't turn out to be good in my opinion. I even saw a comment saying they would really love to see Ginga bottom for Kou... and when they read it, they said that it was okay but unnecessary... like I guess they didn't expect it to be like that. It would've been good if their desire isn't because of Yuuzen. Maybe just them mutually in love with each other. Idk, but I totally just agree with you

The extra ruined the whole story for me. It just feels kind of odd that they hid it from Yuu. I know they’re poly but it feel likes cheating or at least deceiving.I also liked the dynamic they had before the extra, after it I just felt like Yuu shouldn’t have been there in the first place. I also hated how they seemed infatuated and obsessed with Yuu yet didn’t feel the same way towards it each other? It actually irks me and I feel like they could of just scrapped Yuu and made it Kou x Ginga or kept the dynamic the way it was. The extra soured my mood, there’s nothing wrong with liking it, I just feel like it ruins the story for me, I can’t be the only one. The story itself isn’t even that good and definitely feels like it doesn’t properly portray a healthy and mutual poly relationship.