At Slate on Monday, Yale Law prof Kenji Yoshino proffered the trademark-derived idea that opponents of same-sex marriage fear that extending the term "marriage" to the relationship between two men or two women will tarnish its meaning for male-female couples. Yoshino attributes this sense of tarnishment to homophobia: "Tarnishment claims arise only when the mark is being associated with something uniformly deemed unsavory." Before assuming this motivation for everyone who thinks same-sex marriage diminishes opposite-sex marriage, however, Yoshino would do well to read a column by Stanford Law professor Richard Thompson Ford published in Slate a year earlier (and linked at the bottom of his own piece), and to consider a related but slightly different concept from intellectual property law.
Ford's conclusion that much of the opposition to same-sex marriage is motivated by anxiety over sex roles accords well with my own observation that no one who thinks men and women are differentiated only slightly by genitalia and hormones seems to be bothered by same-sex marriage. In other words, all the opposition I've seen comes from people committed to the notion of a male husband and a female wife, to maintaining distinctions between the sexes. Sometimes this is related to a prioritization of procreation, but it needn't be. For such people, the meaning of marriage, the content of the concept, includes the union of one man with one woman. There can be no impingement on a trademark unless it has some meaning to it; to use one of Yoshino's examples, you can't degrade Tiffany's by using it for a porn shop unless Tiffany's had built up a non-porn meaning.
But I think it's possible to oppose same-sex marriage on this ground without necessarily being a homophobe, if the concern is not so much tarnishment as blurring. That is, the anti-SSMers may not think that heterosexuals are superior to homosexuals (after all, marriage requires no proof of heterosexuality), but they don't want their man + woman meaning for marriage to be blurred by adding man + man relationships to the category. Hence the greater friendliness toward civil unions; if same-sex couples just want the same state-mandated benefits and responsibilities that come with marriage, without using the trademark itself, they're more likely to win over the non-homophobes. They also may be joined in use of civil unions by opposite-sex couples who think marriage actually has negative connotations due to its history as a patriarchal institution, or just because both adultering Henry Hyde and thrice-wed Bob Barr have been part of it. Those who see the call for civil unions as solely a "a cynical electioneering ploy to try to please both anti-gay and pro-marriage-equality forces" are unduly harsh.
Were I living in Quebec or New Zealand, I might well have a civil union in acknowledgement of the fact that I don't have much attachment to religion or traditional sex roles, and therefore am deserving only of the wholly state-created designation rather than one dripping with other meanings. Alas, neither QC nor NZ is on the list of possible wedding sites. I find the UK's refusal to grant civil partnerships to opposite-sex couples almost as silly as their forced mock-marriages.