You could've avoided reading it. I mean in real world, this is definitely not the ideal relationship. If it was p3dophilia or real incest, that would whole another issue. But those who started reading it and stayed got to have expected them to be a couple. At least 98% of BL manga couples at the starting, end up being together at the end too.
The point is why even write and conceptualize this? For ann audience that wants to and enjoys pedophilia and grooming as entertainment as a happily ever after? I know there will be a rebuttal that this is fiction, so what, is there a need for this as “entertainment”? It’s not me clutching my pearls and asking for censorship, if an entire written work centers around romanticizing and downplaying the grotesqueness of immorality is that not just creating media that normalizes the subject matter? There is no critique, there is no nuance, at face value the story is a an adoptive father raising his son to become is lover. Furthermore even if I had engaged with this story fully knowing a father and son would become romantically involved, am I not allowed to point out shortcomings in the narrative? This particular story used the “incest” more as a gag initially and apparently decided to follow through, but its lead up had no warnings, no advisory, no proper labels. Your argument that I could have avoided this easily is moot point when it’s 3 volumes of media before the actual reveal of an undisputed pseudo incestuous relationship. If YOU enjoyed it great, others are allowed to feel otherwise.
To ask why someone would visualize something fictional is honestly redundant. We aren’t the author, but it’s clear with most of these mangas that the author writes it knowing the subject matter is taboo, the characters know it, and the audience knows it. It’s not normalizing it, it’s fetishizing it. Which isn’t a good thing, yes, but what the person before was suggesting was that it’s a choice to interact with it. You have to be dense to have not expected them to end up together. You said the adoptive father raised his son to be his lover, when the entire manga it’s been made clear that he’s been nothing but a good father to his son. Nothing has been implied that he did anything to groom his son. You’re allowed to criticize the narrative choices, but that’s not what you’re doing, you just have an issue with the whole premise. You say it’s a “moot point” but it’s been apparent from the beginning of the manga that his “secret” is that he likes his dad. I’m not sure why you came to a 2000s yaoi manga for a healing family drama.

Even if he’s not your real biological “son”, that is your son you raised since he was 3yrs old. The fact that he accepts his adoptive son’s romantic feelings and even goes as far as having sex with him on a whim is just wow I know this is a considerably older BL but this is fucked up in all sense of the word and this genre really just doesn’t give a fuck pseudo incest be damned.