Matthew 1:20-25 Douay-Rheims 1899 American Edition (DRA)
But while he thought on these things, behold the angel of the Lord appeared to him in his sleep, saying: Joseph, son of David, fear not to take unto thee Mary thy wife, for that which is conceived in her, is of the Holy Ghost.
And she shall bring forth a son: and thou shalt call his name JESUS. For he shall save his people from their sins.
Now all this was done that it might be fulfilled which the Lord spoke by the prophet, saying:
Behold a virgin shall be with child, and bring forth a son, and they shall call his name Emmanuel, which being interpreted is, God with us.
And Joseph rising up from sleep, did as the angel of the Lord had commanded him, and took unto him his wife.
And he knew her not till she brought forth her firstborn son: and he called his name JESUS.
Re-read the last verse here. Joseph did not know (i.e. have sexual relations with) her (i.e. Mary) UNTIL Christ was born.
The perpetual virginity of Mary is unnecessary to fulfill scripture. The scripture was fulfilled when Jesus was BORN of a virgin.
It would also mean that Joseph and Mary did not ever consummate their marriage.
Mark 10:7-8
For this cause a man shall leave his father and mother; and shall cleave to his wife.
And they two shall be in one flesh. Therefore now they are not two, but one flesh.
Agreed. The virgin birth was all about Jesus and His divinity and fulfillment of Messianic prophecy. The "perpetual" virginity makes it all about Mary and has NOTHING to do with Scriptural fulfillment.