Finally, we note that our interpretation of the language of the marriage amendment is one of first impression, insofar as it concerns a relatively unique phraseology. Thus, while other states have adopted constitutional amendments and/or statutes that place limitations on governmental recognition of same-sex relationships, no court in any of these states has had the occasion to interpret language approximating the “similar union” language found in Michigan’s marriage amendment. Consequently, guidance from the decisions of other jurisdictions is unavailing.
Insofar as other states use similar language, and the Michigan Appeals Court notes that Kentucky and Wisconsin do, this decision could end up providing guidance in interpretation, so it's pretty significant. The text of the Michigan amendment, for the record, is, "To secure and preserve the benefits of marriage for our society and for future generations of children, the union of one man and one woman in marriage shall be the only agreement recognized as a marriage or similar union for any purpose."
I may post again later after I've had time to think about the court's reasoning. In the meantime, curious lawyers and fellow travellers are welcome to follow the link and offer their thoughts.