Toward a More Skeptical Monogamy
I've heard atheism and polyamory referred to as "skepticism, properly applied." I'd like to unpack that a little. As happy poly people, we love polyamory. We love polyamory so much that sometimes, it sounds as though we think polyamory is the only way to have a good relationship. So when we say that properly applied skepticism results in polyamory, it might sound as though we mean that monogamy is inherently unskeptical. This is only half true. It is true that if we lived in a more skeptical world, there would be a lot more polyamorous people. If people took a skeptical approach toward their relationships, many people would conclude that monogamy was not the best way to achieve their goals. However, not everyone would. There are plenty of ways to practice monogamy skeptically, and I'd like to go through a few of those.
Both Parties are Only Interested in Each Other
This is the most often-cited reason for monogamy, but often one of the rarest to be approached in a skeptical manner. Most people feel sexual or romantic desire for more than one person. However, not everybody does. A couple who approached their relationship skeptically could easily conclude that they were only interested in each other. However, the difference between this and your garden-variety monogamy is that skeptical monogamy (or what Shaun calls accidental monogamy) would not have rules against outside sexual or romantic connections. They just wouldn't happen, because neither party would be interested. A skeptical couple, however, will know they cannot predict their future desires (especially many years in advance), so a skeptically monogamous couple will not make long-term plans or rules that are dependent upon their desires remaining only for one another.
Both Parties Enjoy a Controlling Dynamic
One of my least favorite things about traditional monogamy is that it involves each party being controlling in regards to the other party's sexuality. It's a form of ownership over an incredibly important part of another person, and I find it repellent. However, some people are into that! And that's totally ok! In the kink community, there are tons of examples of people who have no desire for an egalitarian relationship. There are relationships which are explicitly based on ownership and control. Both parties go into the situation with eyes open, knowing what they want, and knowing what they are getting. This is entirely compatible with skepticism. So long as each party has skeptically examined their own (and each other's) desire and each party enthusiastically consents, this sort of relationship is compatible with a skeptical worldview.
Social Consequences
If we lived in a more skeptical world, this would not be an issue. However, we live in the real world, and in the real world, being polyamorous can have consequences. Often, if we are thinking skeptically, those consequences are less bad or less likely than they seem. But sometimes the consequences are real and relatively certain, and it makes sense to try to fit in. Part of what the community is doing is trying to make this less of a concern, but as it stands now, the fear of societal consequences (especially in less liberal regions/countries) can be a legitimate, skeptical reason to stay monogamous.
Lack of Ability or Desire
This is lumping a lot of things together, but basically it stands for the proposition that polyamory takes a certain amount of emotional work and emotional stability, and not everyone is able to do it, either due to mental illness, societal conditioning, or just plain personality. For most people, being happily polyamorous takes a lot of effort.* A skeptic will not shy away from working toward a worthwhile goal, but there is always a cost/benefit analysis. For some people, the amount of work is too great, the payoff (in terms of happiness) too small, or the chances of success too low. A skeptical approach to life will recognize this and make decisions accordingly. Some people will reasonably conclude that it's just not worth it. So long as their partner agrees, it's a reasonable position.
I'm sure I've missed other reasons why a truly skeptical person or couple might choose monogamy. My intention here was to explicitly acknowledge that monogamy is not always a bad or unskeptical choice. However, I'd also like to stress that the bar for skeptical monogamy is pretty high. It requires a critical examination of all parties motives, desires, predictions, and assumptions. Just as polyamory takes a lot of effort, so does monogamy, and it's not something to be entered lightly. As with all important decisions, it's best to approach it in a critical, skeptical manner.
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*this is another area in which we are hoping society will improve. As polyamory becomes more mainstream and monogamy becomes less of a default expectation, starting a polyamorous relationship should take much less work in terms of switching away from a mononormative mindset.