Marriage is a contract, and government oversees contract law, enforces contract, and settles disputes when a contract is broken or invalid... Many philosophers believe that one of the reasons humans need government and why government is useful to humans and government needs to exist in human society is to maintain social order by enforcing contracts between parties and settling disputes when contracts are broken.
I tend to find the argument that government should get out of marriage interesting for a few reasons.
1. As long as people value the meaning of marriage and want to marry, there will be a divorce rate (or breaking of the marriage contract). I really don't think it's feasible the believe that everybody in this country could settle a divorce dispute without lawyers or a court system. The government will get involved eventually, because as Locke, Hume, and other philosophers say, settling disputes between people and maintaining order is one of the most basic and fundamental roles of government.
2. If you eliminate marriage laws and benefits of marriage, it's radically changing our concept of a family in the eyes of the law. We would no longer have rights to our partner.. their property is separate, their retirement.. although our assets would really be tangled.
If my husband suddenly dies, I wouldn't inherit his property or military benefits, or be covered under any of his employer benefit programs? I mean, if I could... then anybody could, right? .. since I wouldn't be viewed as family in the eyes of the law.
Eliminating legal marriage wouldn't simplify property rights, survivor benefits, or inheritance laws, it will make them super complicated.
3. If you don't believe the government actually plays a role in maintaining order and people can do that themselves without government, then why aren't you an anarchist?