The Perpetual Virginity of Mary is one of the four Marian dogmas of the Catholic Church. Not unlike the dogmas of the Immaculate Conception and Assumption of Mary, in body and soul into Heaven, this de fide doctrine derives its integrity from the first Marian dogma of Mary being the Mother of God, in virtue of her first-born Son’s divinity in his single-person hypostatically united with our humanity. Mary is the mother of God or the Divine Logos incarnate (Isa. 7:14; Lk. 1:35, 43; Jn. 1:14; Col 2:9). So, the dogma of Mary ever-virgin basically holds that the mother of our Lord remained a virgin her entire life given the Divine Maternity, despite her marriage with Joseph and the Jewish religious and cultural norms of the time.

According to Catholic beliefs, Mary was a virgin when she conceived Jesus, during his birth, and after she gave birth to him. Additionally, Catholics have always viewed Mary's union with the Holy Spirit as being similar to a marriage in a spiritual sense, much like the relationship between YHWH and Israel. This spiritual union was considered to be moral in nature. Mary's decision to remain chaste during her entire life and to stay continent in her marriage, regardless of who her legal husband may have been, was guided by the Holy Spirit from an early age. She was chosen to be the virgin spouse of the Holy Spirit in a spiritual and mystical sense from all eternity, as mentioned in Lk. 1:35.

God designed a covenant between Himself and Mary, His handmaid, that was similar to a matrimonial bond. This covenant was endowed with all the dignity contained in the sacrament. It was fitting, therefore, that a partnership that reflected the entirety of life and was naturally inclined towards the good of the spouses, the procreation of offspring and their nurturing, should exist between the two. In the sacrament of Holy Matrimony, a man and woman become one flesh to fulfill the Divine purpose of consummating their marriage. Similarly, the Holy Spirit and Mary should become morally one in spirit, through their quasi-physical union, in accordance with the Divine moral law, for the same Divine purpose. To some extent, through the prompting of the Holy Spirit, God proposed to Mary when she was a young girl living and serving in the Temple.

He claimed her as his own virgin bride and possession, not only so that they should beget the holy Child together, but also that through their consummation they might beget all His children who would be regenerated unto God by being reborn in the Spirit (Jn. 3:3; Rom. 8:29). All members of Christ’s Mystical Body are the only other children Mary begot following the birth of her divine Son, who belong to the spiritual family of God that transcends all blood ties in the natural world (Mk. 3:31-35) and are the offspring of the free promised woman (Gen. 3:15; Rev. 12:17). - MC

He did not have marital relations with her before she gave birth to a son.
Joseph named the child Jesus.
Matthew 1, 25

Good News Translation

Before she was in labor she gave birth;
before her pain came upon her she was delivered of a son.
Who ever heard of such a thing, or who ever saw the like?
Can a land be brought forth in one day,
or a nation be born in a single moment?
Yet Zion was scarcely in labor when she bore her children.
Shall I bring a mother to the point of birth,
and yet not let her child be born? says the LORD.
Or shall I who bring to birth yet close her womb?
says your God.
Isaiah 66, 7-9

And Joseph also went up from Galilee, from the city of Nazareth, to Judea, to the city of
David, which is called Bethlehem, because he was of the house and lineage of David, to be
enrolled with Mary, his betrothed, who was with child. And while they were there, the time
came for her to be delivered. And she gave birth to her firstborn son and wrapped him in
swaddling cloths, and laid him in a manger, because there was no place for them in the inn.
Luke 2, 4-7

Behold thou shalt conceive in thy womb, and shalt bring forth a son; and thou shalt call his
name Jesus. He shall be great, and shall be called the Son of the Highest; and the Lord
God shall give unto him the throne of David his father; and he shall reign in the house of
Jacob forever. And of his kingdom there shall be no end. And Mary said to the angel:
How shall this be done, because I know not man? And the angel answering, said to her:
The Holy Ghost shall come upon thee, and the power of the Most High shall overshadow
thee. And, therefore, also the Holy which shall be born of thee shall be called the Son of God.
Luke 1, 31-35