A stable environment is the best basis to bring up children - that's as clear cut a conclusion of social science as I know of.
While I am not certain the science is entirely settled on this, from my own experiences, I tend to agree.
Marriage should provide a bulwark in achieving that stability.
Should? Really? Is that the best you can do? Should? I have anecdotal evidence that your thesis is wrong. Does an anecdote prove a thesis? No, but it can disprove one.
My best friend refuses to get married. He now has children who are close to 18 and they have had an extremely stable home so far. Marriage not required; however there is intense pressure for him to marry her. Both families and all of society essentially conspire to force them to marry. In fact, there are laws which say they are legally married now despite him not wanting to get married.
That you seem unwilling to accept the value of stability or the value of marriage towards that stability, I find perverse.
I am unsure how you read that I am against stability or "the children" but go ahead and keep crying that cry. For the children indeed.
That your perception of human nature is so pessimistic, I just find sad.
I prefer to perceive reality, not wishful thinking based on puritanical models of behavior. That you find reality sad is none of my concern. I will give you this advice though: Yourself, and many others in this story have deeply ingrained puritanical leanings. These are biasing your views of what you see going on around you. Beware. Witch hunts are not that far away.
Look at the posts here: Most of them are screaming for blood. That all of the victims here deserved it because they were in turn victimizing someone else by cheating. Some have even called for all of this data to be examined by society.
Be careful what you wish for, you may get it. Be careful of your biases, they will lead you to very dark places.