The answer is yes, polygamy is an attack on (traditional) marriage. However, this then begs the question - is that a bad thing? My answer would be “not necessarily.”
There are four potential “marriage structures” that are worthy of discussion - monogamy, polyamory, bigamy, and polygamy. I think a society that permits and accepts the first three will see much greater success in permanent, happy bondings. And I believe permanent, happy bondings are the key to a moral, productive, and happy society.
MONOGAMY: Traditional marriage falls into this bucket. Inherently, human beings are not monogamous animals. Therefore, forcing this structure universally across our species is not wise and, not surprisingly, this reality manifests itself in the form of incredibly high (>50%) divorce rates in the U.S. (and that doesn’t even account for the portion of the intact marriages that are unhappy). Having said that, invariably there is a portion of the population who desires complete monogamy, so this social construct works for that group of people, just not everyone. (For the record, I fall into this bucket.)
POLYAMORY: For the purposes of this answer, I will define polyamory as a marriage between two people where one or both partners openly have discrete “affairs” (meaning each partner knows that the other is having affairs (i.e., open), but neither flaunts it within the relationship or publicly (i.e., discrete)). I think polyamory is probably the most natural social construct for people. It provides both the stability and emotional satisfaction of a permanent relationship while providing the excitement and variety of new encounters. And these affairs will actually act to strengthen the permanent relationship since they act as an outlet. They provide the variety that humans’ natural non-monogamy requires so that people can sustain the long-term “monogamous” relationship without feeling the need to leave permanently (i.e., divorce).
BIGAMY: For the purposes of this answer, I will define bigamy as one person being married to two other people (e.g., a heterosexual man married to two heterosexual women) or three people all being married to each other (e.g., a heterosexual man married to two bi-sexual women who are married to each other as well). The benefit of this structure is that the variety provided to one or more partners can, as with a polyamorous relationship, help sustain the permanent bonding structure. It may not be as varied as polyamory, but it still may be enough variety for some people to help them sustain a long-lasting relationship. Additionally, each partner can fulfill roles and needs that one of the other partners cannot or does not want to.
Take a hypothetical situation in which a wife loves to shop and the husband hates to. In a traditional marriage, one of the partners is going to be put in an unsatisfactory situation (the husband will have to go shopping with the wife, or the wife will have to shop alone). However, in a bigamous marriage, the two wives can go shopping together (permitted they both like shopping) and the man can go do something he would rather do. Also, bigamy permits a family that has both a dual-income AND a stay-at-home parent structure.
POLYGAMY: For the purposes of this answer, I will define polygamy as one person married to three or more people (e.g., a heterosexual man married to five heterosexual women). Put simply, polygamy should continue to be permanently outlawed. Once a “marriage” has extended beyond three people in total, the motivations for bonding are no longer rooted in love and/or wanting to create a healthy family structure. Instead it becomes solely about the power. Why exactly that is the case is probably the topic of a different post, but history has born out the fact that once a man takes a large number of wives, the situation devolves into a cult-like territorial atmosphere in which the patriarch’s goal is to exercise extreme control over the clan, increase the size of the clan, and possibly confront other clans.
An important note. I’m not saying that there are not downsides to polyamory or bigamy. There absolutely are. Negative forces such as jealousy, complicated logistics, and initial societal taboos will most definitely exist. However, over time, as people acclimate to these new structures, they would learn how to deal with them just as monogamous people learn to deal with the issues that come along with a traditional marriage. On that note, it’s not as if monogamy doesn’t have it’s own issues (50%+ divorce rates, unhappiness, non-consenting affairs). So the point is not to find the perfect solution for everyone. The point is to make a number of viable social constructs available in our society so that each person can pick the one that best suits their disposition in order to maximize the odds of a successful (i.e., permanent) relationship.
If I had to guess what the best societal distribution of the three viable relationship structures is (and this is based on absolutely nothing but a gut feel), I would say: + 20% of relationships should be monogamous + 70% should be polyamorous + 10% should be bigamous