I don't know if I misunderstood you but the seme was cheating on MC/uke since 3/4 years ago while MC never cheated on him.
I think that there were a lot of factors that influenced on MC keeping silent about his illness.
First: Guilt
He thinks that this sickness is his Karma and he must accept it. When they started dating and in the years that followed, he made a lot of wrong decisions in order to help his lover. He didn't do anything bad per see, but those decisions ended up in a bad path for him and his close family.
Second: because he loves him sincerely, even if he suffered in the recent years he still had all those years full of memories and hardships shared with him,
Third: Loneliness
Seme completely isolated him, he didn't have friends (that's why he asked him about what friend gave him the flower) and his family died years ago leaving alone without a single person to relied on. Since the beginning it's clear that he already accepted this fact and trust no one but himself.
Fourt: Pain
He doesn't like pain and would never want someone to feel pain for himself either.
fifth: Giving up
I think that after seme rejected the idea of eating with him (on the first chapter) he gave up on the idea of telling him. Besides for me it also marks the start of MC giving up of"them " and their "relationship"
What I was saying is that the seme though that his boyfriend was cheating on him, so he had to prove that he was the best, he was a man and cheated on the uke without knowing the thruth or without even not wwanting to know. So I blame the seme, the uke did nothing wrong, he only did bad choices, but who hasn't done the same?
Stop making me cryyy!!!!!!
I really hate the seme, but why does the uke don't tell him about his illness? I think the uke is not prepared to tell anyone about his cancer and due to this, the seme started to suspect that his boyfriend was cheating on him, so he startedcheating too because these type of men needs to feel like men and have to have sex with more people to be a bully （￣へ￣）
I hate the fact that the seme told the uke that he was ugly due to his pallidness
ヽ(`Д´)ﾉ I really hate these comments, you can always have trust with your partner, but you should have tact because you can hurt them!
I really want the uke to be with the doctor who really cares for him and not with the seme. You have to suffer a lot asshole!!!