Keith Riler analyzes the Left’s anti-Christian sentiments in “Secularists, or Just Anti-Christians?” at American Thinker. Certainly, as he discusses, their position is inconsistent:

Contradictions are what happens when life is lived relativistically, based wholly on the self and its shifting feelings about the world. This way of life stands opposite the lifestyle based on a higher, objective, and consistent reality. Humility is the key to yielding to and achieving a freedom within objective truth. This is a far better approach to the constant dissatisfaction that comes with the angry denial of attempted autonomy.

Toward the end of the article Riler says:

Finally, secularist hostility toward Christianity may be a juvenile rejection of authority.

Ah-ha! I believe this really gets to the heart of the matter. No one is going to tell them what to do – except them. They’re stuck in the anger phase of adolescence. Trapped at the point where their burgeoning sense of autonomy is shouting “I just want to be me!”. They reject governance of any kind, including self-governance. Think “Rebel Without a Cause” – or Rosie O’Donnell on an incoherent rant.
Sad to think they may never mature to the point where they’ll realize the sense of peace, and real freedom, that comes from submitting to a Higher Authority.