whatever the trope's marginality and extremely high rate of IRL abuse risk, it's still a trope, so the answer to your question is as clear as for any other trope: the exploration of a fantasy.
what's surprising is that the author has managed to turn the dynamic around so that the one being abused is the adult: he repeatedly pushes back the physical closeness initiated by daichi and shows the latter that he's visibly distressed by it, but daichi continues to overstep his boundaries all the same.
obviously, daichi is a teenager, so the responsibility lies with the adult who fails to act sensibly and stop the relationship for good. not (soft-)grooming tho.
did anyone say that you were dumb? did anyone say that it being a fantasy changed something? did anyone say that it being a fantasy was an excuse? you ask a question, you get an answer. everyone is aware that it's fcked up babe, everyone knows that these dynamics are dysfunctional and shouldn't be normalized IRL.
it being a fantasy only means that some people escape their daily lives by reading such horrors in a fictional setting. and what's written here isn't a crime: unfortunately, it's legal in some countries for a teenager to have sex with an adult (it shouldn't be).
the author isn't making that grown ass man the "innocent" one, he's both guilty of wanting a sexual relationship with a teenager, and a victim of the teenager not respecting the boundaries he put at first.
yeah yeah I'll be dropping this etc
but why do they have the NEED to make older characters meeting with the other one when they're a child?
and I'm not complaining about the age gap at all, but this ~soft grooming~ is just... (⊙_◎)