Firstly, I'm not suggesting that open relationships can never work for anyone. Everybody will have different relationship preferences and people should be allowed to consensually do what makes them feel comfortable. However, I believe that a lot of people have been lied to in regards to the viability of open relationships as a result of the positive moves towards sexual liberation that we've made over the last few decades.
The depiction of open relationships in both the media and in wider society has increased steadily over the years, partly thanks to the internet. A combination of content creators, an ongoing move towards open-mindedness, and the increased use of dating apps means that information surrounding open relationships is more accessible than ever.
Consequently, while open relationships might be seen as a relatively niche kind of relationship, COVID-19 actually increased their prevalence. Experts have projected that nearly 50% of couples in Britain turned to open marriages in the midst of the pandemic to alleviate loneliness. Apart from me being personally sceptical of that statistic on its own, it does bring to mind some key reasons why people cheat on their partners in monogamous relationships: disconnection, and loneliness.
We might also view open relationships as a more expanded version of friends with benefits, but even that comparison is not flattering, with 31% friends with benefits ending, with one reason being due to unrealistic expectations and/or unreciprocated feelings on the part of one of the people involved.
So, based on looking at similar forms of relationships and seeing how they end, I don't think that it's unreasonable to suggest that a lot of us are naturally monogamous people. The article about the rise of British couples entering open marriages also mentions something important: it is often the male partner that suggests the arrangement, with the female partner needing time to think about if they consent.
The female in this situation is probably right to be cautious; in the United States of America (arguably the most sexually liberated country in the world), 92% of open marriages end in failure according to a study that was conducted in 2010. While you could argue that this was the result of a combination of social stigma and a lack of education surrounding healthy open relationships, the social stigma of it has not lessened all that much, with research indicating that public acceptance of open relationships in the US (a much more liberated culture than ours) is still negative.
Overall, while open relationships might work for some couples, there are too many factors working against open relationships being either socially acceptable or a sustainable model of relationship. I would only suggest entering an open relationship if you know (and are not just lying to yourself) that you are not the kind of person who doesn't get jealous or insecure at the thought of your partner seeing other people. Otherwise, it will most likely end in heartbreak.