No one forced anyone to stay. In monogynous relationships. He could leave.
Many/most people want a monogynous relationship with just one person. Because they crave that love, one-to-one, the shared experiences, hoping that one person will truely support you alone, have your back. Deepness. It's just you and them. And trust.
Because many humans crave deeper experiences, deeper emotions. Deep, with just one person.
Not everyone wants that. Some people want less commitment, want to have relationships with 2 or 3 or more people. There is no problem with this.
You can shag as many people as you like.
No one is stoping any of us.
But. Once you start misleading/not telling the whole truth. Lying. Trust. It becomes different.
If everyone did the right thing, the honourable thing : faithful and truthful (and we all know when we are in such a relationship with someone, where they expect it and it is expected - rather than a ONS or casual fling) and then if and when didn't feel that, we left. We'd be ok.
But people don't do that.
Really it's a lack of honour and integrity. And selfishness.
And if OW did the honourable thing, not be the OW, that would help too. But they don't.