Does that piece of paper certifying you're married make a difference? I used to think not. And I still don't think it affects a relationship. However, as I grow older I am thinking that the little piece of paper does make a difference.
Yes, someone can still walk away and leave a marriage. Someone can choose to end the relationship. But that piece of paper means they are unable to just walk without untangling themselves legally.
It means that they have to make the time and effort to resolve their finances jointly. It means they cannot just bail and start depositing their paycheck elsewhere. It means they cannot just run away from their families (at least not legally).
That being said, I've always felt that if you get pregnant get married. Because you want to protect your children financially. So should you get married even if you don't have children? Do you have to protect yourself financially? Or is it a fair trade?
I believe that piece of paper gives you peace of mind that if something should happen like death or an accident you also have the legal right to determine the future of your significant other. That you are able to have input on what their wishes really were. The older you get and the longer you live away from home, usually you grow apart from your parents. And they may not realize your values have shifted from burial to cremation, organ donor or not, etc.
So while it still is a piece of paper, and does not guarantee a relationship, it can bring a peace of mind. It can secure your financial future or ruin it. And it can at least protect your interests and values.
But as a bit of background, I lived with my DH for 5+ years before marriage, so I'm not a good example. However, looking back, I wonder if we didn't make a mistake in not getting married earlier? That if something had happened, our parents for sure would have overridden our BF/BG inputs with their own values. But some things you only learn later :)