Just to be clear, immaculacy is not a word normaly used (it means "without stain"). You probably mean Immaculate Conception of Mary. That is the doctrine that Mary was conceived without originall sin. Sinlessness is a separate doctrine, that Mary never committed an actual or personal sin. Virginity is absence of carnal relations with a man. It is believed that God opened the Blessed Mother's hymen to allow for the birth of Jesus, and then he closed it, but that is not a dogma of the Church, just a common belief.
Fullness of Grace is a state incompatible with actual or personal sin. St. Stephen is described as full of grace at the time prior to his martyrdom, although the Greek word describing Mary is different and used uniquely in application to her. The fact that the Angel calls her already filled with grace as he meets her is sometimes used to give a biblical prooftext of her sinlessness as well as her immaculate conception. But we believe that fullness of grace is not something unique to Mary; it is a sanctified condition that pious people achieve with divine help.
The Catholic understanding is that a wife does not sin if she has normal, loving and non-contracepted carnal relations with her husband (and vice versa). On that score alone, Mary would not have sinned if she did the same either. Catholicism never viewed legitimate sex as "dirty". Perpetual virginity of Mary was fitting for her not because she avoided sin, but because she already fulfilled abundantly the union with God by carrying Him, raising Him, serving Him, and believing in Him, thanks to her unique role as the Mother of God.
Let us step back for a while and understand that spousal sex, to a Catholic Christian, is a model and a prefigurement of creative power of God and his love for us. Sex is pleasurable because it is a foretaste of Heaven. We are given spouses to love, and love carnally, so that we can understand and experience loving on the divine level, which through the Incarnation is carnal also. (I trust the good taste of the reader not to make silly caricatures of this). So for that reason sex for Mary would have been superfluous, akin to someone in the middle of a sumptuous dinner dashing off for a quick hamburger.
Joseph is venerated alongside Mary by the Church, as are all the saints, but his role in the Incarnation is secondary compared to Mary. It is Mary and not Joseph who gave Christ his humanity and it is Mary who accompanied Jesus through His minitry and witnessed the birth of the Christian Church at the Pentecost. (It is believed that Joseph was much older than Mary and died at some point before Jesus began his public ministry).
Lastly, what I outlined is Catholic theology. The popular belief through the ages was probably less precise. It is entirely possible that millions of Catholics held some jumbled "culturally influenced view" that a sexual act alone and regardless of context is tainted with sin and therefore incompatible with Mary.
Kosta's spot-on post reminded me that in Mary we see the Tabernacle of the Word, -- or rather, the tabernacle is a prefigurement of Mary. And God had severe punishment in store for anyone touching the tabernacle, -- hence Mary's lifelong virginity.
1. Mary never needed to participate in the Atonement or Grace of Christ.
2. This is a unique human experience outside of Christ Himself in all human existence.
3. Joseph, who died before Christ's ministry began (I've come to agree with this belief), died a failed Jew (no off-spring, no consummated marriage) and outside of the saving Grace of Jesus Christ.
Are all three of those correct/accepted conclusions for Catholics?