This is simplistic. When you get married, you become family. That person becomes your next of kin, with all of the legal rights and obligations. You are also creating one household, with legal rights over the productivity of the household and legal recognition and protection of any children within the marriage. Marriage isn't about getting something from the government -- it is about protecting the legal rights over the fruit of your marriage (be that monetary or children) you have as a spouse. Most people here agree that a person has the right to the fruit of his labor. Marriage creates a unique contract in which your spouse also has a right to the fruits of your labor because you agree to work together for the common good of yourselves and your children. Often this will require one or both of the spouses spending part of their time caring for children, household needs, or other things that create value but don't earn money. Also, you each automatically get 100% rights over all children issued from the marriage. This is necessary, but also different than non-married people who usually have only 1 person with primary custody.
I love to hear people say "I love her, we don't need a piece of paper". Then, for example one case we had at work, a guy's girlfriend is in a tragic accident and coma, and he wonders why her bank account and custody of their kids go to her parents (who of course hate him).
It is possible to achieve most of the same legal rights as a marriage with legal contracts, but it requires a lot of effort and expense. The exception is the tax protections. Within a marriage, you are treated as one unit and can pass money and things back and forth freely without incurring any kind of gift or income taxes. Once one spouse dies, you also aren't subject to any estate taxes. No other relationship is treated the same way for obvious reasons. So in order to abolish marriage, we would also first need a libertarian utopia where all of those taxes were gone.