It depends. Most of the time, it's based on how much has been invested on the relationship and how much there is to lose if you break up, emotionally, mentally, financially, and reputation-wise. It is alot more difficult the higher you are up the food chain. Alliances and relationships with other people will also be strained. I'm not saying breaking up is a bad thing, it's just not as easy to do, the more baggage and connections you have.
There's a theory that you can fall in love with two people at the same time. But it could also be that even though you've fallen in love with another person, you don't want to let go of the other because of memories or circumstances or just being a coward. Human beings are complicated. It's much better to stay single. Lol.
Not everyone views relationships the same way. Tora didn't seem the least bit perturbed by Ane-san's arrival therefore he doesn't see his behaviour as a betrayal or that he hurt her. Ane-san too strolled in and treated Rogi like a pet more than a threat, she's more surprised Tora is sleeping with a man than anything else. However that's just these two, I can't answer your question if we're talking about monogamous relationships, in the end it's about what you're like as a person.
Now Can somebody explain to me why at some point people cheat but still continues with their partners ? Why ?if it's because you lost interest then why not just break up ? If you need each other then why do you hurt each other ? I can only think about that after finding out that anne san knows that her husband cheated multiple times and she is okay with that omfg i just wanna finish this flashbacks