Is it unjust for the State to only give partner benefits to married people? Maybe. But it is rational to allow the state to limit its benefits to long term, committed relationships. If your relationship isn't one that involves marriage, then maybe it isn't the type the state wants to pay benefits for.
Is it unjust for the State to only allow heterosexuals to morry? Yeah, I think so. But I suppose I can understand the viewpoint that the state sets what relationships can be recognized by the government. I'm having a more and more difficult time listening to advocates of discrimination based on sexual orientation; but, I recognize it is not an insane point of view.
Is it unjust for the State to at the same time offer benefits only to married people and tell homosexual employees (and all homosexuals) that they cannot marry? Yes.
The case is Collins v. Brewer and here are some knowledgable folks discussing it on Horizon.