Are civil unions literally marriages?

Deborah Widiss and Andrew Koppelman have a short article on federal recognition of civil unions, arguing that federal laws that deal with marital relationships ought to recognize couples in a civil union as though they were married. From the abstract:

This essay argues that the current federal policy is based on a mindlessly literalist approach to statutory interpretation that should be repudiated. Federal law generally looks to state law to determine valid marriages, and the state laws that created these alternative statuses defined them to be fully equivalent to marriages in all respects. Couples who enter into such unions thus make exactly the same legal commitments to each other that couples who marry make to each other. Just as the federal government routinely recognizes foreign marriages, even when called something other than "marriage," it should recognize state civil unions or domestic partnerships that are likewise legally indistinguishable from (other) marriages.

When I wrote about federal marriage recognition in Beyond DOMA, I assumed, like the federal government now does, that civil unions would not be recognized as marriages. But my views on this have been evolving (and here is an earlier post where I started thinking about this).

Now, in fact, I am not convinced that refusing to recognize civil unions is "mindlessly literalist"—it might be worse than that. As Widiss and Koppelman note briefly in the paper, one of the states with civil unions actually defines them as marriage, and many define their participants as "spouses," which is the term sometimes used by federal law. So even a literalist approach would likely recognize civil unions as marriages for some purposes. The current federal policy, it seems to me, cannot even be defended on literalist grounds.