So the guy with the glasses is the bottoms fated mate. When the glasses guy barely bit the back of his neck, at that instant, he rejected the top, because he's not his fated mate or some shit
That's about it, based on my understanding
In-universe, both Saku/Renji (as well as Saku-chan's mom) believe that Saku-chan was rejecting Renji – becaute the glasses guy, despite being a POS and assaulting Saku, is his "fated mate". On a genetic level, they are as compatible as can be, and it's a physical reaction so a person isn't expected to just "snap out of it".
But knowing how clichés work, we as readers can take a pretty good guess(/theorize) that that wasn't the case. Since all the medical team can say is that Saku was suffering from a "rejection reaction", and can't (medically) tell who he was rejecting, there is a possibility that the rejection was instead towards the assaulting alpha (glasses guy) and not Renji.
But as far as the answers the manga has given us so far, and what the characters themselves are basing their actions on, Renji was the one rejected.
im reallyyyyyyy hoping the author is doing a fake-out and Renji is not the one being rejected
Me too! Honestly, all foreshadowing seems to point towards that. But I wouldn't mind if it was indeed Renji being rejected, as long as they do a good job developing the "logic wins over instincs" bits...
So he got Alpha rejection reaction because he got assaulted by someone he doesn’t love? Or the reaction was for the one who separated him from the assaulting alpha? Like the assaulting one is supposed to be his soulmate?