From Catholic Answers:
Fundamentalists’ Objections
Fundamentalists’ chief reason for objecting to the Immaculate Conception and Mary’s consequent sinlessness is that we are told that "all have sinned" (Rom. 3:23). Besides, they say, Mary said her "spirit rejoices in God my Savior" (Luke 1:47), and only a sinner needs a Savior.
Let’s take the second citation first. Mary, too, required a Savior. Like all other descendants of Adam, she was subject to the necessity of contracting original sin. But by a special intervention of God, undertaken at the instant she was conceived, she was preserved from the stain of original sin and its consequences. She was therefore redeemed by the grace of Christ, but in a special way—by anticipation.
Consider an analogy: Suppose a man falls into a deep pit, and someone reaches down to pull him out. The man has been "saved" from the pit. Now imagine a woman walking along, and she too is about to topple into the pit, but at the very moment that she is to fall in, someone holds her back and prevents her. She too has been saved from the pit, but in an even better way: She was not simply taken out of the pit, she was prevented from getting stained by the mud in the first place. This is the illustration Christians have used for a thousand years to explain how Mary was saved by Christ. By receiving Christ’s grace at her conception, she had his grace applied to her before she was able to become mired in original sin and its stain.
The Catechism of the Catholic Church states that she was "redeemed in a more exalted fashion, by reason of the merits of her Son" (CCC 492). She has more reason to call God her Savior than we do, because he saved her in an even more glorious manner!
But what about Romans 3:23, "all have sinned"? Have all people committed actual sins? Consider a child below the age of reason. By definition he can’t sin, since sinning requires the ability to reason and the ability to intend to sin. This is indicated by Paul later in the letter to the Romans when he speaks of the time when Jacob and Esau were unborn babies as a time when they "had done nothing either good or bad" (Rom. 9:11).
We also know of another very prominent exception to the rule: Jesus (Heb. 4:15). So if Paul’s statement in Romans 3 includes an exception for the New Adam (Jesus), one may argue that an exception for the New Eve (Mary) can also be made.
Paul’s comment seems to have one of two meanings. It might be that it refers not to absolutely everyone, but just to the mass of mankind (which means young children and other special cases, like Jesus and Mary, would be excluded without having to be singled out). If not that, then it would mean that everyone, without exception, is subject to original sin, which is true for a young child, for the unborn, even for Mary—but she, though due to be subject to it, was preserved by God from it and its stain.
The objection is also raised that if Mary were without sin, she would be equal to God. In the beginning, God created Adam, Eve, and the angels without sin, but none were equal to God. Most of the angels never sinned, and all souls in heaven are without sin. This does not detract from the glory of God, but manifests it by the work he has done in sanctifying his creation. Sinning does not make one human. On the contrary, it is when man is without sin that he is most fully what God intends him to be.
The doctrine of the Immaculate Conception was officially defined by Pope Pius IX in 1854. When Fundamentalists claim that the doctrine was "invented" at this time, they misunderstand both the history of dogmas and what prompts the Church to issue, from time to time, definitive pronouncements regarding faith or morals. They are under the impression that no doctrine is believed until the pope or an ecumenical council issues a formal statement about it.
Actually, doctrines are defined formally only when there is a controversy that needs to be cleared up or when the magisterium (the Church in its office as teacher; cf. Matt. 28:18–20; 1 Tim. 3:15, 4:11) thinks the faithful can be helped by particular emphasis being drawn to some already-existing belief. The definition of the Immaculate Conception was prompted by the latter motive; it did not come about because there were widespread doubts about the doctrine. In fact, the Vatican was deluged with requests from people desiring the doctrine to be officially proclaimed. Pope Pius IX, who was highly devoted to the Blessed Virgin, hoped the definition would inspire others in their devotion to her.