This question, which was well-phrased, reminds me of an interview in which the interviewer's question was not so well-phrased: "Raghav and Pallavi are now facing a challenge from Mandar who even tried to kill his mother for a woman - I mean, I thought, what kind of man does that? What is your personal view?" My impression is that she was asking, Mandar is so unscrupulous; how will Raghav and Pallavi fight back? But Sai Ketan Rao answered as if she was asking, what do you personally think about matricide? "It's only a fictional show, but I think it is very bad."
Likewise, my opinion is: of course, adultery is bad when it's a betrayal. Monogamy isn't for everyone, and I don't want to shame anyone's sexual expression, but if you promise to be faithful to someone, you should mean it and treat your partner with respect. Some couples renegotiate their marriages (e.g. you can openly visit your paramour, but we'll raise the kids together), and other couples say that cuckolding brings them closer. I feel sorry for anyone who feels pressure to compromise on fidelity to keep a relationship going, but I can also respect someone who doesn't cheat in secret but says, "I want more out of life, so let me out of my promise," and deals with the consequences.
I have no single or simple answer for what I would do personally; it would depend on my quality of life with or without the cheater. I can too often tell when someone is being dishonest with me (e.g. falsified scientific evidence, plagiarism, flattery, overcharging), and sometimes I have to confront on principle, but if I can manage not to make the cheater cry, I have a better day.
As for a fictional show, my motivation is escapism. I enjoy pretending that there are characters who are too good for infidelity. For example, on Sundara Manamadhe Bharali (a Marathi daily drama), when Kamini stole the maṅgalasūtra that Abhimanyu was going to give to Latika, and told Latika that he gave it to her, Latika just scoffed, "Śakyaca nāhī (just not possible)," and kept walking. Latika had no idea that her husband had fallen in love with her, and she thought Kamini might be his type, but she felt confident that he would never behave shamelessly. That depth of trust in someone is magical. Another dialogue that I found magical: Nandini said to Abhimanyu, "Just to give Latika a jolt, I told her, if you actually divorce Abhimanyu, there will be girls lined up for him, and don't be surprised if I'm at the end of the line!" and Abhimanyu's reaction was, "There isn't any girl going after me." He knew very well that Kamini is making a fool of herself for him, but in his world, only Latika desiring him is significant.
While I find thoughtless betrayal distasteful, I won't pretend that I'm opposed to a married character looking outside for a strong reason. I'm writing a fan fiction in which Pallavi leaves Raghav and says yes to Mandar, who is secretly gay and desires Farhad mutually, but wants to protect Pallavi from Raghav. I'm curious - does anyone find my story obscene?