Well that's because these people are incapable of critical thinking so contradictions don't occur to them, everything is viewed in isolation and nothing interacts with anything else. Anyway this isn't so much about telling the world what they think, it's about trying to pretty up their abhorrent worldview.

At some level they realise they've got to be at least a bit progressive, so they throw out support for gay marriage as a sort of fig leaf, because they really do think that gay marriage is still a subject matter that needs further public litigation (it isn't).