In another universe...

Now imagine the same premise, but with more powerful drama. The mc is about to die from a very rare disease, and his only chance to be saved is to be f*ed by some stranger dude, who's accidentally drunk the antidote due to doctor's mistake. Mc desires to live, so he hasn't any other choice but to ask seme to be with him until he's cured. Seme realizes his mistake and, driven by sense of guilt, eagerly agrees to help our mc. Moreover, seme is gay and mc appeals to him, so deep down he's even a little happy for this accident.
They start "going out", mostly trying to know each other better, and soon they develop warm feelings for each other, something more like friendship, but not quite love yet. Nevertheless, later they start even live together and their life seems to be pretty ok, though mc is still in death danger and greatly depends on seme.
It's said that mc needs a "dose of antidote" every two weeks, and if he misses even one, he can die in terrible agony within the next two weeks. "The antidote" helps a lot, but the chances to be cured completely are super low. So both guys are very cautious about the matter and try not to be separated for too long. It's not the problem for them, because they've already established almost family-like relationship and just enjoy their strange, but fluffly lives. But then something happens, and a small fight occurs. It's just a fight, but for short period of time they left apart from each other. It's then, when both of them realize, how fragile and odd their relationship actually is. Mc knows he cannot sulk forever. But not only because he wants to make up - he starts to be afraid of seme leaving him to die. Seme understands too, that if someday he himself finds a real partner or falls in love with someone, he couldn't built a relationship with him, because he still needs to have sex with mc. Seme feels guilty, but at the same time he doesn't want to be bind to mc only because of this "antidote". Does mc even love him in the first place or is he just afraid to leave?
When seme comes back to give the "antidote" to mc, the later, terrified and in great suffering, almost pleads him not to leave anymore. Which only supports seme's assumptions.
Then fluffiness slowly turns into angst. Mc decides to find another way to live without the antidote, or another antidote or something else. Seme doesn't know how to act near mc.
...But there's happy ending, of course. I bet, when they find another way to save mc, or when mc is suddenly cured, they could built a real relationship. And they live happily ever after, the end *w*