Q. If monogamous people have to restrict themselves to just one partner in order to be monogamous, how come polyamorous people can still be poly even if they only have one partner or no partners?
The definition of polyamory is not "Must be in a romantic relationship with 2 or more people at all times." Monogamous people are also still monogamous even when they have no partners. It's about the *kind* of relationships they prefer, desire, or have the capacity to have, not a requirement on the number of partners people must have at all times. That a why it's still poly even if someone only has 1 partner.
A straight person is still straight even when they're not in a relationship. A bisexual person is still bisexual even when they only have partners of one gender at the moment.
And everyone is still whatever they are when a relationship breaks up and new relationships have not yet been found. It takes time and effort to find compatible partners. Just because someone happens to not know anyone compatible for a relationship at the moment (even if that "moment" lasts a long time, like years), it doesn't change *who they are as a person*.
There is also a difference between what a *person* is, and what a *relationship* is. A poly *person* is about the kind of relationships they prefer, desire, or have the capacity to have, while a poly *relationship* is about the kind of other relationships that the people in this relationship are available to have by the nature of this relationship's configuration.
A relationship is a Thing with needs and limitations all by itself. If a *relationship* is open to its participants having other partners, but any of the *participants* is not open to having other partners at the moment, the *relationship* is still poly. For instance, a relationship can be open and poly, but maybe someone in the relationship is polysaturated and doesn't have time for any more partners. That individual person not being open to more partners doesn't make the *relationship* less open.
Or if a relationship is open to its participants adding more partners but someone is in the middle of their doctorate program and also working to put themselves through school and maybe doesn't even have the time or emotional resources to maintain the one partner that they have - that *relationship*, and even that *person* can still be poly, they're just tapped out of resources at this moment in time.
And if a relationship is open to its participants adding more partners but one of the participants simply *does not* prefer, desire, or have the capacity to have multiple loving, romantic relationships, this can be a mono person engaging in a poly relationship - the *relationship* is open to that person having more and to the other people in the relationship having more, whether any individual wants to or not. Just as a poly person, who prefers, desires, or has the capacity to have multiple loving, romantic relationships, can *choose* their behaviour to limit themselves to a monogamous relationship.
Polyamory is not Pokemon! Go - we are not here to "catch them all". With all the other things going on in our lives, we can self-limit the number of romantic partners that we have to whatever functions best in our lives and still be poly in nature, just the way that straight people who are not dating anyone right now because they want to focus on these other important things in their lives are still straight even when they're not currently dating anyone.
Monogamous culture, at this point in time and in this region, sees "dating" like an interview process. This allows people who prefer, desire, or have the capacity to romantically love only one person at a time, to *date* more than one at a time (a behaviour, as opposed to a preference), providing that they are in the process of winnowing them down. It's OK (says monogamous culture), to interview a bunch of people at once, because the goal is to optimize your time by dating a bunch of people in order to find The One out of the interviewees. They are trying to identify *which one* of the group is The One they will love forever.
We even have several popular television shows with this very premise - The Bachelor goes out on a bunch of dates with a bunch of women, but his goal is to find out which one of the 20 (or whatever) is his Soulmate and pick The One out of the crowd. These people are monogamous, even though they are deliberately dating multiple people at once.
It's because of the *kind* of relationships they prefer, desire, or have the capacity to have. The multiple dating thing is a *vehicle* to eventually get to the type of relationship they ultimately prefer. Whereas, for polys, the multiple dating thing isn't a tool to get to the preferred type, it's the point all by itself.
Or, they may be between "serious" relationships but still enjoying sexual encounters. This kind of "dating around" in monogamy also doesn't include big-L Love, so it doesn't "count". Although, this kind of setup *is* debatable among monos - some would not count this as acceptable within the Monogamous Paradigm.
Behaviour and internal inclination are not the same thing. Everyone behaves in ways contrary to their natural inclinations all the time, and for a lot of different reasons, many of which include social cohesion like following traffic laws when we'd rather drive faster or asking permission or paying for something before taking it rather than just taking it because we want it. We can make choices for our behaviour that do not necessarily align with our preferences. It doesn't change our preferences.
When gay people are so closeted that they don't even admit it to themselves, they often go their entire lives in hetero marriages, become biological parents to children with opposite sex partners, etc. That doesn't make them straight. It makes their *behaviour* appear to be straight, and they can have any number of reasons for choosing to do this, including fearing for their lives.
Polyamory is not about how many partners you have. If that was the only criteria, then we wouldn't need swinging, RA, monogamish, or any of the other labels of non-monogamy that's out there. Technically, we wouldn't need the word "polyamory" because we had other words for multiple partnerships long before polyamory came around. We came up with the term precisely because we wanted to differentiate between the *kinds* of multiple partnerships that we were having and the *kinds* of multiple partnerships that other people were having. It was never about the numbers, it was about what those numbers represented.
It's about the *kind* of relationships that you have. One of those important criteria is how the person in question handles *their partner* having other partners. If they prioritize a primary couple and insist that their partner only have casual sex with people they meet at parties intended for hooking up with casual partners, and they only have sex with the hookups together as a couple - that person probably isn't poly, they're probably one specific type of swinger. But they have multiple partners, so it's not about the numbers, it's about the nature and the criteria of those partners. If they revel in their partner's autonomy, encourage their romantic interest in others, view metamours as potential opportunities instead of competition, and feel compersion, that person is probably poly even if they aren't romantically involved with anyone else at the moment.
They could also be any number of other things, like a swinger, a kinkster, a sex worker, etc. - one person can enjoy, prefer, or desire different kinds of relationships. That makes them all of the labels, it doesn't disqualify them from all of those labels. Some are mutually exclusive - so one can't be both a monogamist and a polyamorist at the same time, but most of them are not mutually exclusive; the different types of non-monogamy labels just clarify a certain type of non-monogamy that a person can like, and just like a person can like different kinds of desserts, a person can also like different kinds of non-monogamy.
There are a lot of things that go into whether or not a person is poly or mono, not just how many people they're dating. In my opinion, the actual number of partners at any given time is the *least* important of the criteria to determining if someone is poly or not. How they feel about metamours, what *kinds* of relationships they prefer to have, their ethics on interpersonal dynamics - all of these things are more important.
Someone looking at a relationship group from the outside and only counting the number of people can't tell any of that stuff, which is why we can't label other people's relationships without their input on themselves.