Firstly, id say that I am using unethical in a technical sense – that is an action which is not ethically justifiable (pardon the tautology, but the definition should suffice) is necessarily unethical.
based on the commonly accepted ethical principle "you shouldn't risk hurt other people for no good reason if you can help it, acting otherwise is unethical" (which i'm willing to justify meta-ethically if i have to, but i'd rather the discussion didn't go in that direction):
here's the argument.
1.) For a great many people, when he or she finds someone he or she loves has had sexual intercourse with someone else, it causes them pain.
2.) some people have experienced emotional trauma that leads them to cognitively devalue their own right to sexuality and happy romantic relationships, and as a result will not report accurately about their own feelings to a romantic partner, or even to themselves.
3.) it is possible for such people to be polyamorous and have polyamorous relationships.
Therefore, even if an agent S is emotionally equipped to be polyamorous, S can never be sure that S's having sex with someone other than a specific partner is not causing a specific partner pain for no good reason and by way of actions that S can control.
Therefore, polyamory is unethical.
now, obviously there are exceptions to some of my premises. obviously, there are people that S has less of a responsibility toward no matter how much they feel about you. someone S doesn't love, for example, or someone that S is only seeing casually and who hasn't informed S of the depth of her feelings for him. so lay that aside in the evaluation of the argument and assume that this is a case, not an uncommon one, where S is in a committed and mutually loving relationship with a specific partner P who fits the description laid out in premises 1, 2, and 3 above.
please feel free to debate if you so feel inclined
