...So... Why can't he marry her? Why did this virus in him mutate and spread regardless? What, did he have a child with another woman or cheat or something? In which case... just don't do that? I'm so fucking confused how they were married and together in the future and despite that it didn't matter. How is "creampie her and put a baby in her womb" saving humanity while "marry her and potentially creampie her and put a baby in her womb" does not?
...This makes no fucking sense. I can't help but wonder if this was a recycled plotline by the mangaka for an eromanga they never did. "Yeah, this is where he then would rape her, but obviously I can't portray that. I'll just make it they need to have babies. But not get married, because that will still end the world. Because that makes sense."
And if he was really the cause of the virus outbreak... why the fuck would you time travel to the past to specifically get him to fuck his childhood friend but not marry her? Why not just, I dunno, kill him and incinerate the body?