Okay first, let me preface with, sure it’s “wrong” in any manner of speaking by western morality. Let’s move on from there.
First, he knows that line. He’s already straddling it in some way by coaching the JK whom he knows has it not for him.
Secondly, it’s already accepted at this point that the grandfather has “accepted” him as a arranged marriage partner for his granddaughter the JK even tho he’s 29.
Thirdly, the MC has all but accepted it, maybe he has accepted it, and himself has said in this chapter the temptation is so much he is having trouble holding himself back.
Sudden emotional breakdown COULD do that to a person. (Not that it makes crossing that moral line right).
Fourth, he’s dealing with the sexual abuse/harassment at work already for a Kouhai. While this isn’t an “excuse” for what appears to be all but certain assumption of events here, it does go to breaking down Yariba’s common sense and ability to withstand the incoming emotions.
Fifth, while the grandmother “trusts” Yariba not to go to far, she’s already also stated “it wouldn’t be completely bad”. These things in wealthy families are dealt with internally, right OR wrongly. The girl wants to be with him, the guardians approve, the man himself is willing to take responsibility for his actions at this point... all they need to do is wait for the minimum approvals age with guardian approval for the “marriage”. (There is controversial line I won’t editorialize on here for very obvious reasons). Gotta keep in mind the culture mindset as seen by some in Japan... it’s more likely that it’ll all be swept under the rug. As long as she doesn’t end up pregnant.
Sixth, he has this challenge at work where it’s not for sure where he’ll win. In fact he can feel both feet well placed in the ditch out the door. More ‘reason’ to just give in to that possible “downward spiral”.
The man is definitely grasping at straws to stay sane, and in the meantime breaks through on another front regarding why he quit writing...
... I was gonna say something else but now I forgot... lol