From a Buddhist perspective, I do not think marriage is spiritually important.
We should have unconditional, boundless love for all organisms. That's an aspirational statement: In reality, it's tough to maintain!
In practice, we find ourselves really caring for those individuals who we have met. We fall in love. We experience all those hormonal and neurochemical changes, all the life changes that come about from courting someone. These are beautiful parts of life, and a person who has learned to let go of moments and let them flow like water can enjoy them with no regret.
Marriage is a beautiful institution that represents a bond between two (or more) people. But to me there is no magic to the moment after "I do" that goes beyond the actual lived experience. And that feeling of magic can actually mislead us into thinking that a marriage doesn't need trust, and hard work. For many people, that magic fades after about thirteen years (which makes sense from a sociobiological perspective: bonds that last just long enough to bring a child into this world). After that, you need real long-term compatibility.
I personally cannot wait until I am married. I am excited to spend my life with someone. But that's because, despite my anarchist predilections, I'm a true knight on the inside and I don't want to ever be without my Dulcinea.
I don't care much if someone is "shacking up", or in a polyamorous relationship. What I want to see is that their interactions are safe, sane, consensual, and comfortable. I want to see them growing as a person rather than shrinking. I want to see them being more protected rather than less. I want to see people relate to each other in ways that are healthy instead of being addictive, prudent instead of pathological.
There's no magic greater in this world than the moments that we already get to experience. It doesn't need to be there. It doesn't fit.