Mary an Immovable Object?

An old logical puzzle is: what happens if an irresistible force is applied to an immovable object? This is often applied by children who will ask: "Can God make an object too big for Him to move?", where God is the irresistible force. The standard answer (which comes, I think, from Lewis' Mere Christianity) is that God can do any thing, but that question is nonsense.

So, could God have brought His Son into the world through anything less than a sinless virgin? The Catholic version of Jesus Christ is that no, the Saviour is too great to have been born of anything less -- the idea itself is simply nonsense.