First, let me state that I absolutely and unconditionally accept the dogma of the Immaculate Conception. But I ran across a Protestant argument that made me say hmm: They say the dogma was not necessary (which Ineffabilis Deus also implies, even using the word fitting, instead of necesary), and is merely a pius thought. They say that perhaps Our Lord having been born to a sinful woman would have been even more fitting and more humble.
What is the Catholic response to this argument?