
BUT that was the seme choice... He was the one who suggested helping him to forget about the other guy and he is aware that this is not easy, after all he loved that friend for 5 years and he didn't even know the seme that well. And they're just sex friends, the seme didn't confess that he likes him so......

When people are in deep pain and stressed, they can be rather slow to things happening around them, as well as becoming rather selfish, so the story portrays a rather realistic situation. They only see their sadness and pain, while becoming blind to the pain of others. And the uke thinks that the seme is only after sex, since it does seem that way.
I'm kinda pissed off that he wants him only when the other sunbae breaks his heart, he's just using him.