I believe that people's views are shaped by... Almost everything.

Every movie, TV show, book, comic, song. It's almost always a husband and wife, boyfriend and girlfriend. Or a single person looking for their "other half". Or someone in a monogamous relationship who is cheating. Or getting cheated on.

Monogamous relationships in media can either be the focus or just a background detail. Non-monogamous relationships are almost always treated as gimmicks. TV Shows like Big Love or Sister Wives. Harem animes. That kind of thing.

I am really struggling to think of media that portrays non-monogamy as just a normal thing. Maybe media where casual hookups are a thing might count for you? But I see that as a person being fundamentally single usually, not polyamorous.

And there's also incidental media. Advertisements, packaging, etc. The existence of "family size" implies a standard sized family.

Beyond media, there's all of the other examples they have. Parents, grandparent, aunts, uncles, cousins, teachers, coaches. I think everyone has a subconscious push to copy what they have seen elsewhere in relationships, and it takes a good bit of self-reflection and purposeful change to get out of that.

For me personally, my awakening was when I read Stranger in a Strange Land as a teen. The whole book got me in the habit of analyzing relationships of all kinds. That included a section of the book where the main character starts a cult, and the whole cult is basically one big polycule. It was just a small section of the book and it was presented as so... Normal.

We tend to think of "marriage" as like a combo meal at a fast food restaurant- it's the #1, a burger, fries, and a drink. A small house, a white fence, a shared bank account. Someone to be with spiritually, emotionally, sexually, politically, financially, nutritionally (I've seen veganism lead to divorce), and in so many other ways. But it doesn't have to be. Some people sleep in separate beds, or separate rooms. Some people get married but keep separate houses or apartments. Some people keep separate bank accounts.

Even then, I read the book as a teenager and was kind of open to the idea, but never thought it would work in practice. Eventually I started dating my wife who is bi, and the topic of threesomes came up. We ended up having a successful one with a friend of hers she had a crush on for a while. We briefly looked into trying some exhibition, so same-room sex with other couples. Found some people online and went to dinner with a few but never really materialized.

About a year ago another couple we were friends with propositioned. They were both bi, had been monogamous for roughly a decade, and wanted some gayness in their bedroom. I'm like, mostly straight, but for me this presented a lot of rare opportunities for new experiences. So now I have a boyfriend and a non-binary girlfriend in addition to my wife.