"Massachusetts Supreme Court Orders All Citizens to Gay Marry," went the satirical Onion headline (in The Onion, the punchline always comes first). But if "gay marriage" is the current reading of "civil unions," The Onion may be on to something. I start with the premise that the government has as little business as possible in my domestic arrangements, love life, and so forth. As proponents of "traditional" (read: gay-excluding) marriage like to point out, marriage is supposed to be a sacred trust, and government has no business delineating what is sacred and what is not.
Why, then, ought we to tolerate any secular laws regarding marriage at all? There are important reasons, the most important being the adjudication of child welfare and custody, care of the elderly and disabled and visitation rights, and settlement of probate (property, payments, inheritance) in the event of death, divorce, or abandonment. Innocents need protection and money matters need formal procedures, in short. Establishing formal civil status is necessary for people who share dependents or property. Civil union is the right phrase for this arrangement. Note that many couples could benefit from civil unions: elderly siblings who share a house, say, or otherwise unrelated parents of a "love child." I see no reason why civil union could not be undertaken by groups of people larger than two, for that matter.
As for love, marriage, family, romance, sex: the government has no business in any of that. These are matters for individuals, families, churches. Jesse Jackson once pointed out that we can no more coerce someone not to pray in the schools than we can force them to pray: prayer is private and personal, a part of our thoughts and sentiments, not our public actions. It simply cannot be legislated one way or the other. Marriage is like that. Passing laws forbidding people to be married is like passing laws requiring people to belong to a religious faith (the Spanish Inquisition was one large-scale attempt; their enforcement history is instructive).
The irony here is that it is precisely the sacred, inviolate character of marriage, that aspect that causes uneasy intuitions about gay marriage in some traditionally-minded folks, that makes gay marriage (literally) impossible to prohibit. It's like trying to prohibit people from believing in God.