Marriage is two things.
(1) A cultural/religious idea
(2) A contract
Insofar as marriage is a contract, the state has the role of enforcing it, as with any contract.
The state has no role in marriage as a cultural/religious idea.
What does this mean in practice? Say two lesbians want to enter into a contract to hold their property in common. Should that be allowed? Sure, why not. The state should enforce that contract as it would any other contract of any kind between anyone. Say the lesbians want to refer to themselves as married. Should that be allowed? Sure, why not. Naming things oddly doesn't hurt anybody. Say the lesbians want to force the Catholic Church to call them married. Should the state enforce that demand? Of course not, as that would violate the church's property rights (not their "religious liberty," which thing does not exist, except as a sub-set of property rights).