Whether we like it or not, we cannot easily divorce law from culture. The state's legal definition of marriage affects the public understanding of what marriage is and what its purpose is; it serves as a public affirmation of a society's values. Think about it: why were same-sex marriage activists unsatisfied with civil unions that granted all the legal benefits of marriage but weren't called marriage? This is because gay couples weren't just after a set of legal benefits, but public approval of their way of life. Because the state has an interest in promoting the common good and because it unavoidably has an affect on mores, it ought to promote a proper understanding of marriage, which is crucial to the raising of future generations. Catholics and all men of good will must oppose the redefinition of marriage because of its faulty understanding of human nature and harmful social and moral effects.

True, it is by revelation that marriage can be understood as a sacrament and an imitation of the relationship between Christ and the Church; however, the Church has also affirmed that the "natural" understanding of marriage, as an essential societal institution joining man and woman oriented toward procreation and family life, can be attained by reason alone. A glance at other societies around the globe and in history can confirm this. Before the year 2001, not a single government on the planet recognized same-sex marriage, and many countries that do not adhere to any Abrahamic faith, such as China, Japan, and India, define marriage as a union of a man and a woman.

I am skeptical of the idea of the state "getting out of the marriage business." Firstly, given its effect on culture and interest in the common good for future generations, it is a perfectly legitimate exercise of governmental authority to publicly acknowledge marital unions. Second, I am worried that the government no longer publicly affirming marital commitments (even if that includes some adulterous "remarriages" or same-sex "marriages") will only exacerbate the weakening of the norm of commitment altogether. For a case study, consider France, which has not ceased recognizing marriages but now also includes civil unions for heterosexual couples called PACs. More and more young couples are choosing PACs over marriages, and those in PACs tend to be much more unstable and end in break-up much more frequently than marriage.

Tl;dr Pretty sure Catholics can't in good conscience support the legal redefinition of marriage, and (in my own personal opinion), the state "getting out of the marriage business" seems like a dangerous idea.