Some conservative libertarians oppose expanding legal marriage to include same-sex marriage. (For example, see here.) Like the author in the link, a number of them have made the following argument:

1. Marriage establishes number of positive rights and privileges by government fiat.
2. Positive rights and privileges established by government fiat are unjust.
3. If 1 and 2, then expanding marriage would increase the total amount of injustice.

This has a number of uncomfortable implications, including:

A. Governments use to forbid miscegenation: opposite-sex marriage between members of different races. In eliminating miscegenation laws, governments became less just, not more.

B. Governments provide public schools. Suppose a government announced, “We will provide schooling only to white children, because the Bible says that blacks are the corrupt children of Cain.” These conservative libertarians would have to bite the bullet and say that while the government’s rationale here is sickening, this would be a step in the right direction.

If you think government-centric marriage is just a series of Rothbardian-rights violations, and if you think justice is all and only about stopping such rights violations, it seems like you’re committed to A and B. I take that to be a good sign that justice is about more than just Rothbardian-rights violations.