The State of Minnesota was quite clear in the negotiation of this contract, that it would only allow same sex partners to receive domestic partner coverage. It was an easy enough thing to throw into the contract, because it mandates coverage for only a handful of people. The State's reason for not extending coverage to opposite sex domestic partners is, there are so many of them that the cost to the State's insurance program (the State has it's own insurance program for state workers covered by the contract) would be prohibitive.

Is it discriminatory? Yes. But not because of the sexual orientation issue. It is discriminatory because it refuses to extend the benefits married domestic partners enjoy to unmarried domestic partners. The discrimination has to do with the sacred "institution of marriage" and is considered by conservative and fundamentalist politicos, bureacrats, religious leaders, etc. much more dangerous than recognition of same sex couples.

It is discrimination based upon marital status, and there is a considerable amount of it out there.

BTW, Minnesota does recognize common law marriage if the couple began their common law marriage in a state which recognizes it. But I don't think the legal case to be argued in this day and age is for common law marriage to be recognized in all 50 states. I think the legal case to be argued is that unmarried domestic partners are being discriminated against based upon marital status.