Also, God chose to have Our Lady be a married virgin because it was still a real marriage even though both parties intended from the beginning to remain virgins. They both gave their conjugal rights to the other but instead of using those rights they preserve the others virginal chastity. This even today is called a Josephite Marriage, when the spouses get married and choose to remain virgins.
Tradition has always maintained that Our Lady made a vow of virginity and the only reason she agreed to be espoused was that Joseph agreed and confessed to her that he also wanted to remain a virgin. St. Bernadine of Siena states that one of the main purposes that St. Joseph fulfilled was to be a smokescreen to the devil who never thought that the virgin who would conceive and bear a son in Isaiah 7:14 would be a *married virgin*.
Father Elias Mary, FFI
Virginity of Mary and Joseph
Both from Sacred Scripture and tradition we find the evidence that Our Lady had made a vow of virginity.
First of all, the passage from Luke state that the angel Gabriel was sent by God to a virgin betrothed to a man named Joseph and the virgin's name was Mary. The other point is that when the angel Gabriel said to Our Lady that she was going to conceive a child, she responded that how can this be since I know not man. This has always been understood that it was a state that she had chosen. In other words, I have chosen to never know man.
In other words, she was telling the angel that she had a vow of virginity. She was testing the angel to see if he was from God or a devil. If the angel had said, “Well, Mary, you and Joseph are going to have to have relations like everyone else,“ then she would have known that it was not an angel from God since God would not ask her to break her vow to Him. The angel gave her the only answer he could give to satisfy her and that was that it was going to be an act of God and not of Man.