How would you respond to someone who says: “Mary cannot be without sin because the Bible says ‘all have sinned’ ” (Romans 3:23).

It's important to understand context, but first to read the whole thing St. Paul is making reference to. This is the trouble that comes from so many people picking out verses, or parts of verses to the exclusion of the argument to not only make a point, but in a case like this, that small piece of what he's talking about is twisted to build an entire (Protestant) theology.

St. Paul does not make this statement in a vacuum, he is speaking of peoples as a whole, or group. He says that there is no distinction in sinful behavior between those Jews who remain and only abide by the law, those Jews who have lived under the law, but now live under the fulfilment of the law, Jesus (Mt. 5:17-18), and those who had never followed Judaism, but have come to believe and follow Jesus. This is his assertion; it is not written as an individual indictment. This can be seen in the visit of the Magi at Jesus birth (give, or take several months) who sought him out and payed homage to him having never even heard a single prophesy. Their epiphany came and they humbled themselves before him just as any sinner must do. Sin is a universal quality mankind as a whole possesses. That God might make an exception for his mother is not only entirely his prerogative, but to me is entirely appropriate.