So this is basically a cuckold edging manga. The thing I hate about stuff like this getting popular is that authors may start using similar (shitty) story beats in more popular manga. Old 80s manga could get weird but it was usually campy and cartoony now its just turned into fetish porn.
I feel like if you gave an old man the chance to be a horny teenager again (literally) he would take it without question. This "She's hot but I'm an old man" trope is annoying. At least the author lays it right on the table beforehand basically telling the readers there will be no romance.
I'm guessing this will end with him devoting his life to Eril till his death and reincarnating or returning to our world and meeting her reincarnated self
Its impressive how much the author is pushing the character who is the "yuri girl because mc is a eunuch" trope. Usually they are just joke characters and never really in the story much.