Try reading in context.
If you (rightly) answer "Paul did not mean to say that God had sinned", then why didn't he say that?
If you (rightly) answer "but his readers knew what he meant: he didn't need to list the exceptions" then - aha! We have finally got to the point.
Paul's text is not to be used as a delete *.* command.
Paul's words cannot be taken to mean that Mary had sinned, anymore than they can be taken to mean that Jesus had sinned.
Paul knew the awesome title given to Mary by the Angel Gabriel, just he knew the titanic reality of Jesus Christ, her Son.
His context in the passage is that we all need the Salvation of God. The difference between the rest of mankind and Mary is that she received God's salvation from the moment of her conception.
She needed it. And because of her vital role in God's plan of salvation, she was given it at the moment of her creation.
Hope this was helpful.