?

Because he’s a lion and a lion girl in his class was in heat, so it affected him

I’m not sure at 100%, I also didn’t read this carefully as I didn’t liked the lion and how he slept around with anyone but I’m still here to see where this story will go..I read the main story but I don’t really remember well and I don’t know if my explanation can be correct so this is just a theory
So for the heat it can be because they are animals, as a female animals can let out some pheromones when they go in heat, an example it can be an omega’s heat and no alphas can control their instincts

So has the seme become a pair with the uke? Cause usually in abo world you can’t be affected by pheromones after you mated. Not sure if this applies to this animal World

This is not omegaverse, they have animal characteristics and their heats are different (the uke even talks about in the latest chapter of what he heard about a lions heat, cause his heat is different). For now they’re just boyfriends, I guess. I don’t know if they have a mark or pairing system like in omegaverse, but from what I got from the story it’s not like that.
In the first story the problem was that when they were children they were friends, the uke started to like the seme but he didn’t notice, when he hit puberty the seme started having urges and was seduced by a girl and the uke saw them together and they stopped talking. The fact that they are carnivore and herbivore was brought up when they started talking again, because they have separe classrooms for carnivores and herbivores. Supposedly, I guess herbivores are expected to mate with herbivores, and carnivores with carnivores, since there’s a fear of being “eaten”, that was also the problem with their friends relationship (the bunny and the wolf).
There’s a moment when the seme helps the uke with his heat, this is the first time that the uke helps the seme. But they never talked about some type of pairing or mark. And in order for the uke to be pregnant it seems that there’s a special medical procedure needed (he talks about it in the first chapter when they are in the shrine with their friends).
Also, in the first story, they talk a little about what’s expected of them in the future, since the uke is a thoroughbred, in order to keep his lineage he’s probably expected to mate with another thoroughbred (the new guy in the 2nd chapter).