One of the many stock fallacious arguments against same sex-marriage is the slippery slope argument in which it is contended that allowing same sex-marriage will lead to allowing polygamous marriage (or at least bigamy). The mistake being made is, of course, that the link between the two is not actually made. Since the slippery slope fallacy is a fallacy, this is obviously a bad argument.
A non-fallacious argument that is also presented against same sex-marriage involves the contention that allowing same-sex marriage on the basis of a specific principle would require that, on the pain of inconsistency, we also accept polygamous marriage. This principle is typically some variant of the principle that a person should be able to marry any other person. Given that polygamous marriage is supposed to be bad, this would seem to entail that we should not allow same-sex marriage.
My first standard reply to this argument is that if different-sex marriage does not require us to accept polygamous marriage, then neither does accepting same-sex marriage. But, if accepting same-sex marriage entails that we have to accept polygamous marriage, the same would also apply to different-sex marriage. That this is so is shown by the following argument. If same-sex marriage is based on the principle that a person should be allowed to marry the person they wish to marry, then it would seem that different-sex marriage is based on the principle that a person should be allowed to marry the person of the opposite sex they wish to marry. By analogy, if allowing a person to marry any person they want to marry allows polygamous marriage, then allowing a person to marry a member of the opposite sex would also allow polygamous marriage-albeit only to a member of the opposite sex. But, if the slide to polygamy can be stopped in the case of different-sex marriage, then the same stopping mechanism can be used in the case of same-sex marriage.
In the case of different-sex marriage, there is generally an injunction against people marrying more than one person at a time. This same injunction would certainly seem to be applicable in the case of same-sex marriage. After all, there is nothing about accepting same-sex marriage that inherently requires accepting polygamous marriage.
In light of the above, the polygamy gambit against same-sex marriage would seem to fail. That is, the claim about the slide into polygamy that would supposedly result from legalizing same-sex marriage is unfounded.
There is, however, still an interesting question in regards to polygamy, namely the matter of whether or not it is wrong. After all, even if it could be shown that same-sex marriage would lead to polygamy, this would only be a problem is polygamy was actually wrong in some relevant way.
While polygamous marriage is not unheard of and there are also traditions of the practice, appealing to common practice or tradition to defend polygamy would obviously be fallacious. What is needed is a proper examination of the practice.
It is often the case that polygamy is condemned not directly because it is polygamy, but because of other factors associated with the specific sort of polygamy in question. For example, a culture that accepts polygamy might do so based on the view that women are inferior to men. In this case, it would not primarily be the polygamy that is problematic, but the way women are regarded and treated. As another example, polygamy might be practiced with under-aged and coerced brides (as has been seen in certain cults in the United States). In this case, the main concerns would seem to be with the coercion and age. In these and similar cases, the main point of concern would seem to not be that a man has many wives, but the treatment of the women. Thus, the moral problem with polygamy might not be a moral problem with the polygamy aspect, but the context of the polygamy.
Let it be supposed that polygamy was occurring in a situation devoid of such other negative factors. That is, those involved were not coerced, underage, or mistreated. The question would then be this: what is it about having multiple spouses itself that is wrong, if anything?
It might, obviously enough, be countered that any polygamous nature would be defective. For example, it could be argued that polygamy, by its very nature, must involve an imbalance in marital power (usually the male over the females) or, at the very least, it would always result in some of the spouses being denied the full benefits of marriage (that is, a single man could not attend to the emotional and physical needs of multiple women).
Naturally, it can easily be pointed out that critics of “traditional” marriage have pointed to the traditional imbalance in power between men and women and women being denied the full benefits of marriage. As such, these defects could be defects in marriage rather than a defect specific to polygamy-a polygamous marriage might merely multiple the disparities.
It is worth noting that these defects seem to arise from polygamy of the traditional sort: a male possessing a harem of wives. As such it would seem worthwhile to consider various forms of non-traditional polygamy, especially one involving multiple spouses of different sexes. Naturally, there could be different-sex polygamy of this sort (the marriage holds between the different sexes but not between the same sexes) or same-sex polygamy or bi-sexual polygamy. The notion of an extended marriage (with co-wives and co-husbands) was considered in science fiction by Robert Heinlein and he seemed to regard it as a potentially healthy and effective system of marriage. Of course, the fictional consideration of this matter could, at best, be considered a thought experiment. However, Heinlein did note the advantages for children (which he seemed to be regarded as of great importance in the context of marriage) in terms of the number of parents available to provide care and support.
Obviously enough, we have no real evidence of how a polygamous marriage between free and equal spouses would actually work-we just have our unfree and unequal world to draw upon for examples. However, it should, perhaps, not be dismissed out of hand or regarded as inherently defective.
In response to the obvious question, I would not want multiple wives. I failed with one wife and have no desire to multiply my failure.