The Wedding at Cana
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Harry Chalmers has claimed that monogamy is morally impermissible. His argument goes as follows: we would find it morally problematic for a couple to prohibit one-another from making additional friends. This is because “Friendships are an important human good, and when we’re in a romantic relationship with someone, we should want our partner to have such goods in her life” (Chalmers 2019, 225). But “Sexual and romantic relationships are themselves an important human good” (ibid., 225). So why do we not regard it as morally problematic for a couple to forbid one another from pursuing additional sexual relationships? “Is disallowing one’s partner from having additional partners any better than disallowing one’s partner from having additional friends?” (ibid., 225)
The challenge, then, is that of “finding a morally relevant difference between [the] restriction on having additional partners and a restriction on having additional friends” (Chalmers 2022, 1009). Here is an obvious proposal: perhaps the human sexual faculty has innate procreative and unitive functions, and these functions render non-monogamous sexual relationships morally problematic. It would plausibly be immoral to deliberately go around having children by multiple partners, and hence polyamory cannot properly respect the procreative function. In a similar way, to engage in sexual relations with multiple partners is to disrespect the sexual faculty’s unitive function. Pace Chalmers, this is not analogous to the claim that having only one child makes the parent-child relationship more special, since our concern here is with an objective, mind-independent function of the relevant faculty, not merely a subjective sense of “specialness.” It is also clear that these considerations would not apply to ordinary, non-romantic friendships, since in these cases there is no procreative or unitive (in the relevant sense) element.
Natural law theory, then, is able to provide a plausible account of why monogamous sexual relationships are permissible (and indeed, why non-monogamous ones are impermissible). Given that this is an extremely widespread moral intuition, it seems that we have here another reason to favor natural law theory.