I recently came across this illustration of an experiment (which may or may not be apocryphal — please comment if you know). In short, a group of monkeys were conditioned to expect that when one of them attempted to eat a banana placed out for them, the rest would be punished with a jet of water. They soon learned to stop each other from approaching the bananas. The interesting part came with the gradual replacement of the population, such that no monkey who had ever experienced the water jet was still present in the group, and yet the group continued to prevent each other from approaching the bananas as they had learned to do from their predecessors.
Meg Barker borrows from Terry Pratchett’s analogy of the crab bucket when talking about something similar to this. The idea is that crabs rarely escape when thrown into a bucket together because each time one tries to climb out the others pull it back down. She points out that social groups can work similarly, with anyone straying from the norms of that group finding themselves subtly dragged back into line by those around them.
When discussing the normal rules of monogamous with people who are new to poly, I frequently find that fundamental objection boils down to “it’s wrong because we know it’s wrong.” I am not actually devoutly poly and I think in many cases there are good reasons for monogamous relationships but those reasons are often not the actual motivation behind people’s relationship choices. Even when you’re following the rules, doing so unquestioningly can lead to as many problems as breaking them. In fact I’ve seen countless examples people sticking to the letter of a rule while horribly abusing the spirit of it, particularly in that grey area of tacit agreement on what counts as “cheating”. They can be as adamant that they are doing nothing wrong as they are that I, with my sordidly honest approach to multiple relationships, am.
The essential point is that rules always exist to serve some purpose. If we don’t question what that purpose is, we don’t necessarily know whether we agree with that underlying purpose or indeed whether those rules actually serve the purpose well. While it would be exhausting to question every rule we encounter, and undesirable for everyone to ignore any rules they didn’t understand, a lot of the time we as a society seem to treat the questioning of rules as tantamount to breaking them. As with the crabs and the monkeys, it doesn’t actually serve our interests to automatically pull back on anyone seeking to challenge the group’s boundaries. There may be a better world out there, or at least a bigger bucket and some tasty and surprisingly consequence-free bananas.
B.Minstrel 
