It's almost as if.. that's the goal
I have small brain, explain
The animal disease gives them extremely strong desires for each other. It makes them impose their feelings of lust and ownership on their partner. When not affected, there extremely courteous towards each other, not wanting to cross any boundary.
So the main conflict of the story is: They truly love each other, but expressing that desire is imposing due to their disease, so even if they do love each other, they think they can't act on it. Kamiya decided after 13 chapters of maneuvering around the issue, that their lives for each other is worth it, so he finally takes the step to express his love, even if it makes him selfish. It's both a selfish action and a selfless action, because while it imposes on Hitomi, it also clears all doubt from this heart, that Kamiya doesn't feel the same way.
Oh yeah no that's obvious but I mean more like it's selfish that Kamiya is pushing away Hitomi by not being together with him, as a way of protecting him but...
like he has made it very clear that he would never and can't break up with him so keeps trying to make hitomi do it instead of him that's the selfish part I am referring to.
Since the beginning of the story and before hitomi has the disease its being shown how down right desperate he is to stay by kamiya's side, to the point of proposing of being sex friends. So the way he sorta keeping pushing the whole "break up with me before it is too late because I cant" is pretty cruel and selfish
Seme comes across as so weirdly selfish.