We'd do everyone a favor if we stopped telling people growing up that monogamy is the natural state of things, and only the odd fuck up fails at it. Truth is, we aren't designed for it. It's a relatively new state of affairs tied more to practical and financial concerns.
It's funny how much of our pop culture is built around this bullshit lie. Movies, books, tv, songs, etc. talking about finding the one, the love of your life. These stories conveniently don't come back 5 or 10 years later to check on that true love. We're wired to want to fuck other people. Intense romantic feelings at the start of a relationship are like being on a cocktail of drugs, and are designed to fade over time. We're less financially dependent on each other than in past generations, and have way more options to find partners, including people that are geographically far away, which used to be implausible.
The odds are really against it, and that's without assuming you're an American male with any money or property. In that case it's a goddamn minefield. Spend 5 years or 20 with someone, there's no predicting if they will or won't just leave you for Chad while taking all your stuff. Even keeping it common law doesn't save you anymore. And the funny part is guys in the exact same situation that got luckier, will fall over themselves telling you it's your fault for picking the wrong person.