There was this girl I used to know in her early 20s at the time. She and her boyfriend had migrated from Bulgaria to find better jobs and have a better life. She worked near minimum wage. It was still far better than she would have earned in her home country. All she had was her boyfriend, they shared a flat and had a dog.
He was seemingly happy with their relationship yet they were a young couple sleeping in different beds. The relationship from her perspective was out of necessity not love. She wanted to leave but she didn't know how. There was nobody that could help her. She never earned enough to leave because they had bills to pay.
This man I know that worked at the same place just started talking with her at work. At first she brushed him off saying she had a boyfriend but that didn't put him off. He just wanted someone to talk to at work. He asked for her number and she gave it to him but she didn't message him back at first. She seen that he was content with just having someone to talk to at work. So eventually she messages him. She appreciates the attention she doesn't get from her boyfriend.
These two start spending all work break times together just chatting and getting to know each other. Then they have lunch breaks at his place. Then she's staying after work. She explains to him about her relationship and he wants to help her out of the situation. She starts visiting his on the weekend and this is likely where she is unfaithful. He gave her the opportunity to leave that she didn't have before. They arranged to make it official and she left her boyfriend who no doubt was heartbroken but couldn't see the relationship failed long ago.
They moved in together and have visited each others family. About a year into their relationship they had a child together. She'll now be about 2 years old. They're both happy in their relationship together. I don't think cheating is the right option but I also don't think you can dismiss everyone's story based on one fact. Life is full of nuance. The devil is in the details.