I think you're probably right OP, but then I don't think there's a "one" you love.
I think you spend enough time with (almost) anyone, and you learn to love them. That love takes different forms, whether it be familial, friendship etc. But you add intimacy and sexual attraction, and you get romantic love.
Most people I'd say get married in their late 20's / early 30s. It's a time when people are looking for stability, to have kids, to settle down, so it's the logical time to get married, but that doesn't mean that they love the person they're with at that point any less than they'd love anyone else in their life.
Personally, I'm not married. I've been with the mother of my child for 15 years but neither of us has ever really seen the point of marriage (other than the financial, which can be sorted in other ways). Based on probabilities we probably wouldn't still be together without our daughter. Neither of us had had a relationship that lasted more than a year at that point, so without the extra influence of a child its likely our relationship would have gone the same way.
Does that mean I don't love her? No, of course not. I've spent almost every day for 15 years with her, we care about each other deeply, I don't want to be apart from her.
I don't think people really "fall" in love. You fall in lust, and not with a person, with an idea of a person. Love is something that is built, it's shared experiences, shared pain, shared happiness. You're building a bridge between two people, and sometimes that bridge is strong, and sometimes it collapses.
If you're building that at a certain point in your life, then for many people marriage is a way to strengthen that. But it's not a guarantee.
So yes, I think marriage is about a life stage, not a person. But that doesn't mean the person you're marrying isn't the right person.