This turned into a massive wall of text and I apologize. I hope it's off some use, at least.
If you ask five poly or monagam-ish people about this topic, you're going to get at least six different answers. Every relationship is different, every person is different, and when you have people in multiple relationships at the same time you're going to get a bunch of different viewpoints from the same people. I don't have a Ph.D in Trout Studies (
...yet), so I won't try to analyze the characters exhaustively and I'm instead just going to stick to real world, general explanations.
First off, how you love is how you love. If you don't love the way other people do, that doesn't make it wrong. If everybody is an equal in the relationship, there's no coercion or manipulation, and nobody is being hurt, then that's Good Love. It doesn't matter if there're two people, ten, or just you and cake (I don't judge). Sometimes when someone talks about the kind of relationship that satisfies them, it can make others feel a little defensive if their relationship doesn't match that, but that shouldn't be the case. If your way of expressing love requires monogamy? Groovy! And if it doesn't and you're happy holding your partner's hand while you're both under an orgy pile, also groovy!
Modern western society has given us a very specific view of how love is
supposed to work. One
cis man, one cis woman, stickin' the P in the V, etc. People who are all about LGBTQIA+ and gender equality can deal with dropping the gender and sex requirements for love. Non-monogamous relationships aren't wildly different from this, as they simply drop another requirement: that there be two and only two people.
Most people have more than one friend at a time. We don't generally expect people to only have one friend. You can love your BFF and spend tons of time together, but that doesn't mean you can't also hang out with other people, right? Just because your BFF is so awesome and your favorite person in the world doesn't mean you don't occasionally want to hang out with other people. And if your BFF wants to hang out with other people, that's no insult against you. Maybe your BFF is a knitter and you get itchy at the thought of yarn, or your BFF is super into foreign films and reading subtitles gives you migraines. You don't expect your BFF to fulfill all of your social needs and your BFF doesn't expect you to fulfill all of theirs. If you saw two good friends who also had other friends, that wouldn't make you doubt their friendship. Some people might only have one friend in the entire world, but that doesn't mean people with multiple friends don't love their friends.
Some people fall in love romantically, experience a strong sexual attraction, and don't want their partner to be with others in a romantic or sexual way. There's nothing wrong with that if both partners agree, but not everybody feels romantic love the same way. There are
asexual people who fall in love romantically without a sexual attraction to their partner, but this doesn't diminish their love. Romantic attraction and sexual attraction aren't the same thing. There are people who may love multiple romantic partners at the same time, but this doesn't diminish their love. Just like with the example about friends above, some people can have multiple close relationships of the same type at the same time. There are people who may love one romantic partner only, but also have sexual relationships with others...and this, too, doesn't diminish their love.
A lot of non-monogamous people would never think of what they're doing as "sharing" their partner. You can't share something that doesn't belong to you. Your wonderful partner whom you love so much is their own person and can do what they want with their own body and life. If they do things that
you don't want, you can end the relationship. This is true for any relationship--mono, poly, swinging, etc.--and what you want and what they want and what compromises you're all willing to make should always be negotiated.
Whether you're monogamous or not, it's a really good idea to lay it all out there openly and look at one another's boundaries, needs, and desires. "I'm okay with you doing Y, but if you do X I'm leaving the relationship" is something we should all consider in our relationships. Some people just have a different Y and X. Maybe you don't want your partner to ever even flirt with someone of their preferred gender(s), or maybe you're cool with them doing that but kissing someone else is your line in the sand. Or maybe kissing someone else is fine, just so long as no genitals come out to say hello. Even if most people identify as monogamous, what counts as cheating to one person is not what counts as cheating to everyone else. So everybody should negotiate, poly or not.
Ultimately, it can be hard to break out of what feels natural to you and what's reinforced by society, but sometimes it's okay to not "get" something entirely and just enjoy reading about it. Neil and Sophie love each other. If they do things that you wouldn't do but everybody's happy and no one's being hurt, then that's Good Love.
