Soredemo Ayumu wa Yosetekuru - Vol. 13 Ch. 166 - The 166th Move

[MangaDex]

 

[Saya2879]

Real life advice:
He should have said no.
It will only create insecurities for his girlfriend and makes everything a bit more toxic.
He doesn't need to cut ties if she stops flirting or trying to seduce him, but he has to if she press on.

Manga life comment:
Eh, who cares, more drama incoming to make the story longer, or simply an author who can't make a rival not love about the MC

 

[JustSomeRandomLad]

Real life advice:
He should have said no.
It will only create insecurities for his girlfriend and makes everything a bit more toxic.
He doesn't need to cut ties if she stops flirting or trying to seduce him, but he has to if she press on.

Manga life comment:
Eh, who cares, more drama incoming to make the story longer, or simply an author who can't make a rival not love about the MC

Here's the thing, she's not his girlfriend. Neither has confessed nor is neither absolutely certain about how the other feels. They are currently in a more-than-friends-less-than-lovers gray zone, so Rin trying to shift his attention to her is completely fine since she's not intruding on their relationship.

Though I do agree with what you said about him putting his foot down and saying no, not because of his relationship with Urushi, but because, at this point, he's kind of leading Rin on with a conviction that he won't fall for her. This honestly feels dick-ish to me, even though she's the one who asked.