The law clerks at the 9th Circuit are having too much fun.
We need consider only the many ways in which we encounter the word “marriage” in our daily lives and understand it, consciously or not, to convey a sense of significance. We are regularly given forms to complete that ask us whether we are “single” or “married.” newspapers run announcements of births, deaths, and marriages. We are excited to see someone ask, “will you marry me?”, whether on bended knee in a restaurant or in text splashed across a stadium Jumbotron. Certainly it would not have the same effect to see “will you enter into a registered domestic partnership with me?”
Groucho Marx’s one-liner, “marriage is a wonderful institution… but who wants to live in an institution?” would lack its punch if the word “marriage” were replaced with the alternative phrase. So too with Shakespeare’s “A young man married is a man that’s marr’d,” Lincoln’s “marriage is neither heaven nor hell, it simply purgatory,” and Sinatra’s “A man doesn’t know what happiness is until he’s married. By then it’s too late.” We see tropes like “marrying for love” versus “marrying for money” played out again and again in our films and literature because of the recognized important and permanence of the marriage relationship. Had Marilyn Monroe’s films been called How to Register a Domestic Partnership with a Millionaire, it would not have conveyed the same meaning as did her famous movie, even though the underlying drama for same-sex couples is not different. The name “marriage” signifies the unique recognition that society gives to harmonious, loyal, enduring and intimate relationships.”
Via Perry v. Brown.