Well, they can’t have their cake and eat it too.
If they don’t cover unmarried “domestic partners” of the opposite sex, then they can’t be intellectually honest, and say they demand coverage of same for same sex partners. Now that same-sex can get married, they need to get married to get these benefits. What’s fair is fair.
I would say that it is more than intellectual honesty - it would be discrimination to provide coverage for unmarried gays but not unmarried heterosexuals.