Most definitions of polyamory are pretty good, but leave a lot open to interpretation, and that has led to a number of assumptions, not all of which are entirely coherent, one of the most problematic being that it's OK to use polyamory to spice-up otherwise boring monogamous relationships, particularly traditional Western marriages.
In western culture and law a marriage is defined more or less the same way. Using Canada as an example, marriage is defined as, "The lawful union of two persons to the exclusion of all others." Consequently from a legal perspective, forming a romantic union ( especially a sexual union ) with anyone other than your husband or wife is considered to be adultery, and that forms legal grounds upon which to grant divorce. So right away there are ethical problems.
In addition to the legal problems associated with using polyamory to spice-up traditional western marriages, there are often religious ones, the most common of which is some sort of pledge to the Judeo Christian God to abide by the rules of marriage approved of by the church. Breaking this covenant clearly represents a serious ethical compromise of a theological nature.
But even if we dismiss the legal and theological problems, there's still the common situation that affairs that take place during monogamous relationships of any kind ( including common-law ) often happen in secret. This adds a layer of lies and deceit to an already ethically compromised situation.
Unfortunately the problems don't just end there. Even when there is no deceit, there is a significant inequality of status between a married couple and whomever they decide to bolt-on to their relationship. In essence they are simply using others as disposable sex toys who can be discarded without consequence when they're tired of them. This hardly seems fair or equitable and therefore cannot justifiably be seen as ethical.
But what if the person being used is OK with it? Unfortunately that still doesn't alleviate the ethical problems. Inequality still exists, and adultery is still hypocritical whether everyone agrees or not. Both of these things violate the spirit of polyamory. Therefore using polyamory to prop-up a boring monogamous marriage isn't the most ethical way to resolve the issues.
What then are those who have found themselves in this situation supposed to do? Logically, to relieve the hypocrisy, the thing to do is to get a divorce and work out an equitable contract between everyone involved. This solution has some challenges as polyamory, though not illegal, is weakly supported by the legal system, with case law and contracts rather than statute forming the basis for most legal decisions. So the legal status of polyamory presents a whole other ethical dilemma.
If we ran the world we'd abolish marriage as a legal concept, allowing people to enter into contract relationships as allowed by perfectly adequate laws that already govern other forms of legal partnership. - Hardy & Easton 1
Should people be expected to compromise the benefits afforded by legal Western marriage in order to live in harmony with their beliefs about relationships? What if the benefits that would be lost by getting a divorce include medical or pension benefits that contribute significantly to quality of life? What if it means becoming a pariah within one's family or social network?
There's no one correct answer to such questions, and no way to avoid ethical compromise. Probably the cleanest way to deal with the problem is to simply avoid marriage in the first place. However that is often not as easy as it should be because Western socialization and social conditioning begins at birth and is constantly reinforced by the media, schools, legal system, religion, and prevailing cultural tradition.
By the time we realize that the system we've been born into is an inherent part of the problem it's already too late for many people to avoid the traps. In many cases people never come to the realization that the system is part of the problem. So they blame themselves or their partners as if there is something wrong with them.
Then the system exploits that false belief via legal, social, and religious institutions that continue to reinforce the same self-serving cycle of deception, reward, and destruction that's responsible for the problem in the first place.