Most people never have to define monogamy. It's the default, so nobody asks you to spell it out, until a conversation with a partner forces the question open. That's usually when "non-monogamy" enters the conversation too, often before either person has worked out what it really means to them. Both words get used very loosely and so it can feel very confusing to figure out. Here's some of the more typical ways to define some of the most popular terms when thinking about monogamy and non-monogamy.
Monogamy means a relationship in which two people agree to be romantically and sexually exclusive with each other, and with no one else, for as long as the relationship lasts. Most social-scientific work on relationships has, until recently, simply assumed this structure as the unstated default, rather than treating it as one option among several (Barker & Langdridge, 2010, Sexualities).
This default status is partly why the definition is rarely examined in practice. One couple might define it purely around sex and not think twice about an ex staying in the picture as a friend. Another might count any close emotional connection with someone outside the relationship as a breach, sex or no sex. A flirtation at a work party is nothing to one person and a serious problem to another.
None of that makes any version more genuinely monogamous than the others. The trouble is that most couples never compare notes on which version they're living by. They assume the word is doing the work of a conversation they haven't had.
Non-monogamy is a broad term for any relationship in which people agree, with full mutual knowledge and consent, to have more than one romantic and/or sexual partner (Gupta et al., 2024, Journal of Family Theory & Review). Everything rests on that word "agree." Non-monogamy is something people opt into together.
Infidelity is what happens when someone breaks an exclusivity agreement without their partner knowing. Confusing the two is one of the most common mistakes people make with this term. Even Britain's own National Survey of Sexual Attitudes and Lifestyles has had to correct for this in its methodology: its most recent wave revised its questions because extra-marital sex can no longer be assumed to mean unfaithfulness, since some of it is consensual non-monogamy, agreed between both partners (Open University, Natsal survey analysis). One is a structure. The other is a breach of trust. They are not the same thing.
Non-monogamy also isn't one arrangement. A few structures sit under the umbrella, each studied and named separately in the academic literature (Barker & Langdridge, 2010, Sexualities):
Open relationships, where a couple agrees that one or both partners can have sexual connections outside the relationship, usually without those turning into separate romantic relationships.
Polyamory, holding more than one romantic relationship at a time, with everyone involved aware of and consenting to the others (Gupta et al., 2024).
Relationship anarchy, which drops the idea of a primary/secondary hierarchy altogether and lets each connection set its own terms. The term was coined by Swedish writer Andie Nordgren, whose 2006 manifesto argued that love isn't a limited resource that has to be rationed to a single partner (Nordgren, 2006, The Short Instructional Manifesto for Relationship Anarchy).
There isn't one clean non-monogamy meaning that can be applied to all relationships, and no single non-monogamous definition covers all elements.
Sometimes navigating these can lead to conflict, and communication is key, we can help
It's tempting to treat monogamy and non-monogamy as a binary, as if you have to pick a side. Real relationships tend to sit somewhere on a spectrum between the two. A monogamous couple might build in real flexibility around a particular ex or a particular friendship. A non-monogamous couple might run their relationship with more rules and more caution than most monogamous ones do.
The label rarely tells you whether a relationship is working. What tells you is whether both people have agreed on the same definition of it, and whether they've talked about it again since life changed, rather than treating something decided once as decided for good.
None of this is an argument for one structure over the other. It's an argument for more precision. Even Britain's principal sex survey got this wrong for decades, treating any sex outside a marriage as infidelity until its most recent wave finally accounted for consensual non-monogamy separately. If the country's largest study of sexual behaviour spent that long collapsing "agreed" into "cheating," it's no surprise two people in a relationship do the same thing to each other and don't notice.
If these questions have come up because something in your relationship feels unresolved, working out which label applies won't fix much on its own. The useful step is saying, out loud, what you want, and thinking about whether the relationship as it currently stands is one you'd choose again if you were starting today. There can be lots of ways to shift and evolve a relationship (monogamy vs non-monogamy being just one of them), most ways to change a relationship however start with a conversation where you both get to explore what you might want and what might be ok for your dynamic. You can ask us questions here
Questions about relationship structure, exclusivity, and what you actually need from a partnership come up regularly in individual therapy and coaching work. Not because our team specializes in relationship structure specifically, but because it's part of the wider question of what a life built around your own values actually looks like. If you're working through something like this and want a space to think it through properly, get in touch with We Are Delphi to arrange an introductory call.
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