
Nope. He won't. There was already a mini comic many chapters ago that depicted their life together in the future. He isn't faithful. Being married in Yakuza isn't about being faithful.
This is actually one of the most realistic Yakuza manga, depicting modern Yakuza anyway. She wouldn't have any recourse even if he had a mistress and children. Similar to the NY-Italian Mafia.

That feels like an alternate universe where she doesn’t return his feelings. The purpose of them dating now is to see if she can develop said feelings. And in the first chapters of the manga, she’d told him that if he did that once she fell for him, her reaction wouldn’t be so blasé if he cheated. As in he would regret it.

Yeah. I did re-read 9.9 just now and it's actually a different translation than what I read before. This one was a little different. Maybe it was a bad joke?
It inferred that they were already married and that he was unfaithful. I recall some translator notes adding context.
I don't mind this translator's version, at all.
I’ve noticed that Kirishima hasn’t slept with any other girls after officially dating Yoshino, maybe he’ll be loyal to her? Am I being delusional?