It feel like you are pushing a specific aspect to the detriment of the whole. I agree that if someone doesn't desire, or can't maintain monogamy in a marriage, that they should be upfront about this and make sure their partner is aware and accepting. While that this may require a bit of social change to make it more acceptable, I don't think that's a specific case that needs to be revisited for marriage in general (many people are happily monogamous). I think it's better overall for people to be upfront and and truthful overall. There are many things that can break up a marriage besides infidelity. I'm not even convinced infidelity is the main reason, it may more often be a symptom of some other underlying problem.
> There's a lot of social pressure against this because of the stigma associated with divorce.
Which ties into above, if people can't (or feel they can't) divorce, then they may act out in other ways. I also think this is highly location dependent. In the western US, I'm not sure I've observed much social stigma for divorce.