[GCC Catholic]

We know that Jesus was born without sin and according to some, so was Mary. If that is true, why would Mary offer a sin offering at Jesus’ circumcision?

Simple:

1) It was required by the Mosaic Law (which because Christ had not yet died, had not been supplanted)
2) It may or may not have been the case that Mary ever realized her own sinlessness (my own speculation), and even if she did, she wouldn't have been boasting about it.

[Dr. Eckleburg]

Protestants believe to assign a sinless nature to anyone other than Jesus Christ is a grave error.

Nowhere in Scripture is Mary referred to as anything other than a devoted mother of the man, Jesus Christ.

Christ even corrects those who sought to assign her some unique position...

"And it came to pass, as he spake these things, a certain woman of the company lifted up her voice, and said unto him, Blessed is the womb that bare thee, and the paps which thou hast sucked.

But he said, Yea rather, blessed are they that hear the word of God, and keep it." -- Luke 11:27-28