In the Denver Post, Tom Minnery of Focus on the Family argues that civil unions are proving to be a slippery slope to redefining marriage:
... Proponents argue that civil unions are no threat to the state's constitutional amendment defining marriage. All one has to do is look to the west to see that's not the case. In 2010, a federal judge ruled that California's attempt to grant civil union-like benefits to same-sex couples created a "second-class citizenship" — and he ordered same-sex marriage as the way to fix the "problem" despite California's state marriage amendment which is virtually identical to Colorado's. That's the same "problem" civil unions would create in Colorado, taking the definition of marriage out of the hands of the people and inviting a federal court challenge like California's.
Every time a state has passed civil unions, demands to the legislature or the courts from gay activists to legalize same-sex marriage have followed — every time. To say civil unions are not about marriage is to disregard these facts.
Colorado voters already rejected a ballot referendum virtually identical to the civil union bill when they voted down Referendum I in 2006. Alongside Ref I on the ballot that year was Amendment 43, which defined marriage as one man and one woman. Voters said yes to marriage and no to domestic partnerships because it represented a counterfeit to marriage, just like the pending civil union bill.