From YOUCAT, fact 83:
83: What does the “Immaculate Conception of Mary” mean?
The Church believes that “the most Blessed Virgin Mary was, from the first moment of her conception, by a singular grace and privilege of almighty God and by virtue of the merits of Jesus Christ, Savior of the human race, preserved immune from all stain of original sin.” (Dogma of 1854 –> DOGMA) [487-492, 508]
Belief in the Immaculate Conception has exited since the beginning of the Church. The expression is misunderstood today. It is saying that God preserved Mary from original sin from the very beginning. It says nothing about the conception of Jesus in Mary’s womb. By no means is it a devaluation of sexuality in Christianity, as though a husband and wife would be “stained” if they conceived a child.