Honestly, I don't know why people get so up in arms about living together. My family basically disowned me (thank God! they are a bunch of whack-a-doos anyways) when I moved in with my boyfriend 11 years ago. Though I was painfully young, he did become my husband and we now are happy and successful in life and in marriage with a child and one on the way.
But, something I will say without getting too over analytical is that marriage *is* different. I can't say exactly why because there are a thousand littleish reasons...the legal protections for yourself and your children, the active commitment to each other that you make in the eyes of the law and the HUGE barrier in place to end the union. Divorce is costly.
Things DO change when you are married. For example, my husband works and I stay at home. If something happened to him, I would be entitled to spouse benefits which is hugely important because I'm not earning money at this time and it may take me awhile to get my earning up to a place where I can support myself and our children. And not only that, as he progresses in his career...let's say we did get a divorce, well, the judge will see the investment that we've made in our lives together and through my commitment to the marriage and to the home and supporting his career and working full time while he was in school, it shows that I'm entitled to a fair portion of our assets and his income for a period of time...like a severance.
So those are the legal and financial considerations, but all of that reminds you there is a lot at stake and it makes you work harder to stay together. It's an additional reason to stay together.
And finally, I think of it like this: If you could, would you pluck a child out of the foster system but choose to not adopt them, just promise to be their parent forever? Is there a difference between adopting a child legally and formally and just informally becoming their parent? Why?
You can love each other and trust each other with everything you've got, but that ring and piece of paper means that even in the hardest times you can KNOW that there was an active decision made that because of love and commitment you will stay together even in the hard times.
But why OTHER people think it's their business?? I have no freaking clue.
I think some people confuse a bad marriage with marriage being bad.