Years ago I wrote an article (in Italian) about monogamy, and how it seldom works: https://psicofelicita.blogspot.com/2018/07/monogamia-non-funziona.html

From all the sources I did find, it looks like just 10-20% of marriages are happy.

During my research, it seemed that a majority (i.e. >50%) of people would eventually cheat over their lifetime (not all the time, not with all their partners). But I did write it six years ago, and I cannot recollect all those sources.

But if you just Google "how many people cheat", you'll find many studies that support my thesis. The first page reports at least 71% of men and 82% of women admitting cheating: https://techreport.com/statistics/lifestyle/cheating-statistics/

Any research reports different numbers, but most go over the 50% threshold. Of course, if you ask people directly they would not admit as much. If your "evidence" is based just on people you know, that's too limited of a sample.

Lastly, I admit I don't know exactly about cheating within marriages, so I corrected "spouse" with "partner" in my last sentence.