Marriage isn't just boilerplate for private agreements. It gives married people legal privileges: custody of children, hospital visiting rights, rights to decide partner's medical treatment in an emergency, tax reductions in some jurisdictions (usually for joint-owned property or for inheritance between the married partners), and so on.

Lack of good boilerplate for marriage-like agreements isn't the problem. In many states, private agreements cannot grant most of these legal rights. That's why abolishing government-defined and -controlled marriage is a prerequisite to equal-rights private agreements.