You are correct. I would suggest that access is already granted from a legal private and or business perspective -the term is partnership -in this case it would in essence be a non-profit private business relationship...
You rightly identify by implication the flawed premise underlying the legitimatizing efforts to institute homosexual marriage being one of sexual activity between the two parties who would 'partner'. Society does not grant two business partners that enter into a contracted partnership special privileges and accommodation if they have sex -why should two that enter into a private business contract be any different?
You carry this posit one step further by suggesting that sexual free partnerships comprised of elderly father and son living together, Mother and daughter, two middle aged ‘friends’ of any sex, etc., are just as valid as the homosexual flavor. In essence, orifice exploration adds nothing fundamentally substantive to the argument for or against...
I suggest you complete the task of carrying the arguments forward to their conclusion and realizing that civil unions are not inevitable. In reality, homosexual civil unions are non profit partnerships that may provide economic benefit to the parties but provide no economic benefit or value to society.
Society has spoken on this as evidenced for centuries in custom, tradition, institution, common law and codified law. The participants in the moral free market have decided that traditional marriage is valued by society while other partnerships although legal provide no value --they are nice, they are legal; however, they are no more worthy of societal privilege, subsidy, and reward than any other business or personal partnership contracted to accommodate and benefit its partners...
The legal and economic status of family relationships - and any other economic or social partnership - has changed a great deal in the last century. For example, marriage is no longer even a contract in our legal system, and efforts to create binding contracts, through prenuptial agreements or attempts to “contractualize” a religious understanding of marriage, have often been thwarted by the courts.
The intervention of government in the economic relationship of parent-child-grandchild, or siblings, or other biological family, has left us with the situation where a sibling can’t insure another sibling, or a grandchild can’t insure a grandparent.
To me, this reinforces the basic premise that economic arrangements should be made by the affected parties, not by government or by government-designated intermediaries (employers, in the case of health insurance). If I want to take economic responsibility for anyone, blood-relation or not, that should be *easily* managed by contract ... but it is not, because government sets the terms, not the private parties involved.