You do not need to rationalize beliefs. Beliefs are just that - beliefs, they are in your mind as well as in the minds of others who share your beliefs. Some people prefer Mac products and always would buy Mac products and that is fine. Everybody can have their beliefs.

To the point of monogamy being the best that the civilization has yielded so far... since it clearly is not the first social arrangement, historically, if it is considered the best, it must be an upgrade, right?

There are several issues with upgrades. First and probably the most obvious one, they are costly. Second, they are usually buggy. The reason some companies' IT departments have a policy not to upgrade automatically but wait and see is that because upgrades introduce bugginess and tracking down problems is costly and time-consuming by itself, to the point of causing serious damage. If indeed 30% of married women per the article from this site have affairs and it is considered bad, then it seems that the model itself is very buggy and failure-prone (30% failure is a high rate) and we should wait and see and let somebody deal with the bugginess, if the issue is simply bugginess, or with faulty design, if there is a deeper issue, before we adopt the model, even if theoretically the model promises to deliver superior results.

Another problem with upgrades is loss of functionality and features. Sometimes you lose so much that you cannot even call an upgrade an upgrade anymore.

Say, Google Drive versus the MS Office suite. Is Google Drive an upgrade? Well, depends on whom you ask and what that person's NEEDS are.

For me personally it is a fantastic upgrade because the thing keeps everything in the cloud, where, thanks god, somebody other than me is responsible for storage (I lose keys, vital documents, forget where I have parked my car, etc. etc. so I just cannot be trusted with storage). Google Docs less feature-full than Word? I do not care. I write for content only and the simplicity of Google Docs is completely fine with me because that massive suite of desktop-publishing-level capabilities of MS Word is not only not needed for me, but is outright intimidating and overwhelming. So if Google drive cannot quite deliver what Word can but timestamps my revisions, spellchecks, stores, makes the content searchable, and is free, I am totally happy with that upgrade. I do not use MS Suite. With spreadsheets, a couple of formulae with vlookups is all I need and the formidable might of MS Excel is excessive for me. So I am fine with Google drive spreadsheets and appreciate the ability to not deal with attachments but have access to the spreadsheets from different computers.

But other people are NOT happy yet because they have needs and requirements that are far more complex. That is why Google Drive and MS Office suite for now co-exist. Will G Drive replace the MS office eventually? We will see; I am sure that MS will make improvements in order to render the arms' race harder to win for their rival.

Going back to monogamy - if you personally appreciate the simplicity and ease of use of monogamy just as I appreciate the simplicity and ease of use of cloud-based office applications, it does not mean that there are not other people who value the more complex functionality of non-monogamy. Why? People have different needs.