You could argue that it’s not the government’s role to enforce morality (which is dubious but a more nuanced argument that resembles this can be made). More reasonable, you can argue that you don’t trust the government to enforce a morality you like, so you’d rather not push moral issues when you have a slim majority and set a precedent that could very easily swing the other way.
You could argue that the government should not even be in the business of issuing “marriage” and thus for political reasons oppose things that “strengthen” marriage as a legal institution.
There are probably any number of others I’m not thinking of. Opposing Prop 8 does not mean endorsing gay marriage or the homosexual lifestyle.
I’m not saying I subscribe to these arguments (as I stated, I supported Prob 8, though I think at this point it would be better if marriage were a religious institution, and the government’s primary involvement was in dealing with the contract involved), but they’re certainly not antithetical to Christianity.