The last few chapters the guy Shiro is with was asleep with 2 other dogs next to him and clearly with no clothes I think
I think the cheating aspect is just a given among the madararui--this is why it's only strange to Norio and Shiro, who are new to the zoomanity. But to answer your questions:
1. He's just sleeping with anybody that would benefit his family honestly. Seems like Kunimasa is doing it to protect Norio and trying to distance from their relationship. Vulnera (or the mermaid guy) sort of explains this in Ch. 48 p. 10. It's a huge responsibility to bear, plus Norio was recently just a monkey.
2. Yonekuni perceives sleeping with Shiro as him losing control (because everytime they slept together, Yonekuni was always dissociative?) so he's probably sleeping with others, again the same reasoning, because he sees it as "protecting" who he actually loves. But the Madarames are clearly fucked up in the head because of it's clan leader.
Ok I’m entirely confused and I keep on reading from the comments about cheating? Straight answers only please-
Who does Kunimasa cheat with and why?
And why is Yukenui or whatever his name is cheating on Shiro? I thought he crave for him?
I mean Kunimasa never really cared for Norio let’s be honest, but what happened?