An interesting observation by legal scholar Eugene Volokh -- that the 9th Circuit's decision validates yet another "slippery slope" argument used in the past by pro-marriage advocates:
...The Ninth Circuit did not decide that all opposite-sex-only marriage recognition rules are unconstitutional. Rather, it concluded that when a state has already recognized same-sex civil unions that are functionally equivalent or nearly equivalent to marriage, denying the symbolic recognition provided by the label “marriage” is no longer rationally related to a legitimate government interest. The court did not decide whether the general constitutional right to marry that applies to same-sex couples, or whether opposite-sex-only recognition rules are generally unconstitutional on the grounds that discrimination based on sexual orientation requires “strict scrutiny” or “intermediate scrutiny” and fails that scrutiny. It only applied the rational basis test, and held that the regime of civil unions but not same-sex marriage lacks a rational basis.
Note that, if the decision is upheld, this means that the arguments that civil unions are a “slippery slope” to same-sex marriage were absolutely right: The recognition of civil unions changed the legal landscape in a way that made it more likely for courts to also conclude that same-sex marriage must be recognized, too.
Perhaps the next time pro-marriage advocates point out a slippery slope, it will be taken more seriously.