I don’t see why Mary had to be sinless to pass on a sinless human nature to Christ.

Mary’s sinlessness derives from the fact that she is the human vessel through which God himself became man. It was from her flesh that Christ received his human nature.

Because Christ is God, it’s fitting that he took his humanity from a sinless human nature, although it wasn’t strictly necessary that his mother be sinless for him to receive from her a sinless human nature. God could have done it another way.

Nor was it absolutely necessary that Christ be born of a virgin. He could have come into the world via the normal route. The fact that Mary was a virgin and conceived Christ isn’t so much a statement about Mary as it is about the dignity of the child she carried in her womb.

Likewise (and ultimately) the Immaculate Conception isn’t so much a statement about the dignity of Mary as it is a statement about the dignity of her son. It points out who he is–God incarnate.