Let’s call it consensual non-monogamy/polyamory (CNM) rather than ethical. Ethics are varied, and asking if ethical poly is ethical is a bit leading. Consent, as I’m defining it means that all partners are in agreement about the terms of their non-monogamy and continuing to give consent through their relationships and any sexual acts in those relationships.
As Dogen said above, there are plenty of instances of non-monogomy in the suttas. Mostly one man with multiple wives, but I remember seeing somewhere in the EBTs a wife with multiple husbands. The Buddha never spoke negatively about them.
The third precept is as follows: AN10.211
They give up sexual misconduct. They don’t have sex with women who have their mother, father, both mother and father, brother, sister, relatives, or clan as guardian. They don’t have sex with a woman who is protected on principle, or who has a husband, or whose violation is punishable by law, or even one who has been garlanded as a token of betrothal.
Kāmesumicchācāraṁ pahāya kāmesumicchācārā paṭivirato hoti. Yā tā māturakkhitā …pe… antamaso mālāguḷaparikkhittāpi, tathārūpāsu na cārittaṁ āpajjitā hoti.
For the first part it’s about consent. Not engaging in sexual activity by one who is protected by their relatives or the law. This would speak to modern day laws about consent and those old enough or mentally capable enough to give consent. This is the part which I tend to focus on.
However, the second half is where we get into the weeds. We have ‘one who has a husband’ - sasāmikā fem. married woman ; lit. with master [sa + sāmi + ikā] where sāmi can mean ‘lord, master, owner or husband’. The explaination then continues with other kinds of relationships where a woman has only one (male) partner.
Most of the suttas run with a male-centric perspective and ask us to flip, so we might imply that whatever the Buddha means here it is not just applicable to those who are women and have male partner(s).
However, we also need to take into account the cultural context of women being owned by their family and then owned by there husband. We can see this in the use of sāmi. There are other words for wives and married. We even have sapariggaha - burdened with wives.
I think that it’s impossible to argue that there was no polyamory in the suttas. How consensual it was, and how much was about family power structures, we don’t really know. I haven’t seen any suttas critising householders for having multiple partners.
How we interpret this in modern times, in a culture where everyone has the right to consent, IMHO leads us back to the first half of the definition. Therefor I think that CNM can be included acceptable in the 3rd of the 5 precepts. However, I think it’s also easy to justify CNM as ‘letting go of attachment’. I’m pretty sure it’s possible to be attached to multiple people and things at once, so this might be a form of delusion.
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Whoever we are with, or if we choose to be celibate, it’s still wise to reflect and review why we are practicing in that way. Our motivations might change and we might see something we haven’t seen before. We should also not be too quick to judge how other people are keeping their sila. Our own conduct is our vehicle for liberation.