Mary remains forever virgin

Concerning the life led in common by the Holy Family, we must stress, and we will freely quote here from the II Vatican's Dogmatic Constitution Lumen Gentium (LG), that Mary remained "semper virgo", i.e. always a virgin:

The Virgin Birth is reaffirmed at the beginning of Lumen Gentium's chapter 8, in reference to the Councils:

[The Son of God] " for the sake of us men and for our salvation, came down from heaven and took flesh of the Virgin Mary by the work of the Holy Spirit" (LG 52).

In conceiving, Mary's virginity is connected to the divine nature of Christ. This physical virginity is also of a spiritual order, for Mary first conceived in her heart:

"She received the Verb of God in her heart and in her body at the same time" (LG 53).

Mary is a virgin while giving birth to the Christ, as is also reaffirmed in reference to the Councils and to St Leo the Great (the latter is quoted in the liturgy). 

"The union of the Mother with her Son in the work of salvation is manifested from the time of Christ's Virgin birth until His death […] At the Nativity, when the Mother of God joyfully presented her first-born Son to the shepherds and to the Magi, a Son whose birth did not destroy but instead consecrated her virginal integrity" (LG 57).

Mary's virginity is bound up with her union to Christ.

She remains a virgin after Jesus is born, as is expressed through the idea of Mary's consecration to God.

The act of giving birth consecrates her virginity (LG 57), and she, from her "Yes" at the Annunciation, consecrates and gives herself entirely to the work of her Son, which is not limited to His coming into this world at the time of His birth but continues until all men have been saved:

""You see before you the Lord's servant, let it happen to me as you have said" (Lk 1:38). Thus Mary, the daughter of Adam, by accepting the word of God, becomes the mother of Jesus, and by espousing the divine will of God for salvation with her whole heart, not hindered by the slightest sin, she gave herself completely, as the Lord's servant, to the person and work of her Son, to serve, in her dependence to God and with Him, the mystery of Redemption, through the grace of almighty God" (LG 56).

Her physical virginity discloses a spiritual virginity related to her spiritual motherhood:

"By conceiving the Christ, by giving birth to Him, by nourishing Him, by presenting Him in the Temple to the Father, by suffering with her Son dying on the Cross, she contributed to the work of the Savior an absolutely unsurpassed cooperation by her obedience, her faith, her hope, and her burning charity, so that supernatural life might be given back to souls. This is the reason why, in the order of grace, she has become for us our Mother" (LG 61).

Her spiritual virginity is a model for all faithful:

"[Mary is a] model virgin and a model mother: it is through her faith and obedience that she conceived on earth the Son of the Father…" (LG 63).

"The Church too is virginal, since it gave her Spouse her faith, which she keeps unsullied and pure; because she imitates the Mother of her Lord, she conserved, by the virtue of the Holy Spirit, in their virginal purity, an upright faith, a strong hope, and a sincere charity" (LG 64).

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(Abbreviation: LG = II Vatican, Dogmatic Constitution Lumen Gentium)