I can conceive of it, but I want to understand it on a deeper level. I think I can safely assume that all human bodies work in the same basic ways. So if someone is not attractive to anyone besides their partner, the question is what do they do with the information about another person that is potentially appealing to them.
I think there is more variety in people than you think. Some people are straight, some people are gay. Then, you might say, the two are still the same in that they're attracted to one sex and not the other. Oh, yes, but there is also bisexuality and pansexuality.
But all these people are attracted to something, right? Well, there is asexuality. But asexuals are still romantically attracted to people, right? Well, not aromantic ones (not that you need to be asexual to be aromantic, either, by the way. There are even cases of people whose sexual attraction and romantic attraction are opposite, for instance only attracted to males but only fall in love with females).
We now know of all these variations. Why not others? Monogamy vs non-monogamy, and within each, subcategories. In monogamy, lifelong monogamy, either romantic (one love ever) or sexual (one sexual attraction ever) or serial monogamy (one person at a time, but more than one over a lifetime, after one another).
Within non-monogamy, you have sexual non-monogamies, romantic non-monogamies, and some that are both.
People obviously do NOT all work the same way. Emotionally we're different from one another. Hormonally we're different from one another. The more you try to find something common to everyone, the more you find exceptions to that.
Lifelong monogamy is certainly not the rule. Even when divorce didn't happen, for instance, people would remarry after the death of a spouse and it was perfectly accepted that they might love the new person.
Although some people do struggle with the concept that it's alright to be in love again when the person you love died, as opposed to the relationship ending in a different one. I don't mean that these people are lifelong monoamorous, I mean that they fall in love again, and feel that it's "wrong" because if their spouse hadn't died, they'd still be there.
Anyway, I'm starting to go off on a rant. My point is that it's often easy to think that everyone works the way you do. I've seen people who claim that everyone is bisexual, just to various degrees, but some people are actually completely one way or the other (and of course asexuals aren't bisexual either). Similarly, some poly people like to claim that poly is the natural state and conventions are the only reason some people are mono.
It's tempting to think that because you had such a revelation (OMG! Poly exists! It all makes so much sense now!) you should share it with everyone because surely it will change their lives too. But no, some people ARE monogamous, lifelong or not. When something seems so obvious about yourself, it's tempting to think it's the way everyone works, but you need to really trust others who tell you they're different, and not assume they're lying or mistaken.
There is a lot of variety in this world, even just about romantic and sexual relations to others.