Are civil unions a threat to marriage?

Civil unions have become popular in Europe, and now they're making headway in the U.S.  Designed to provide a non-marriage alternative to same-sex couples, they're being embraced by opposite-sex couples who reject marriage.  A recent study based on Cook County's civil union law raises some concerns, as John Culhane reports:

Many of the other straight civil union pioneers have also said no to marriage—for themselves and as an institution. The evidence is in a report that the Cook County Clerk’s Office recently issued on the nation’s first opposite-sex couples who civilly united. It found dissatisfaction with the institution of marriage because of concerns with its historical assignment of roles, its connection to religion, and its unfairness to gay and lesbian couples. My own interviews with some of these same couples, who have rejected marriage and plunged into the shallower, murkier pool of the civil union, reflect a cohort prepared to take the wrecking ball to marriage itself.

The numbers in the study are quite limited, and for all we know, those who choose civil unions may never have chosen marriage anyway.  But many defenders of traditional marriage have viewed civil unions as a way to grant legal legitimacy to same-sex unions while maintaining the integrity of marriage.  In the end, will civil unions end up undercutting marriage?