With that comes a ton of expectations and pressure, internally inside the relationship, and externally from society.
These are not present to the same degree with less formal relationships because the stakes are generally considered not as high because there's no formal divorce to go through if it shouldn't work out.
Thus married couples might be more likely to suffer from "performance anxiety" in their relationship compared to less formal relationships.
Different values/desires will always be an issue, but if two people can agree on those, why shouldn't three people be able to? The only reason some are so opposed to this idea is due to old religious stigma enforcing the idea that we have to be monogamous no matter what.