Where does it say “prior”?
It goes without saying as the context shows that this is the meaning.
Luk 1:31 And behold, you
will conceive in your womb and bear a son, and you
shall call his name Jesus.
Luk 1:32 He
will be great and
will be called the Son of the Most High. And the Lord God
will give to him the throne of his father David,
Luk 1:33 and he
will reign over the house of Jacob forever, and of his kingdom there will be no end.”
Luk 1:34 And Mary said to the angel, “How
will this be, since
I am a virgin?”
Luk 1:35 And the angel answered her, “The Holy Spirit
will come upon you, and the power of the Most High
will overshadow you; therefore
the child to be born will be called holy—the Son of God. (ESV)
It's all future tense; the conception hadn't happened and Mary was a virgin
at that point.
It’s perpetual unless you have another scripture that follows it and says she is no longer a virgin
Again, the idea of Mary remaining a virgin has absolutely no basis in Scripture and defies a common sense understanding of human nature, Jewish belief that children were a blessing, and God’s command to multiply and fill the earth. As such, there is no reason to believe that Jesus didn't have half-brothers and sisters through Mary and Joseph. Anything else is speculation.
Therefore, any claim that the Bible must explicitly state Mary did not remain a virgin is unnecessary and it is the opposite that is the case--any idea that Mary remained a perpetual virgin must be shown in Scripture.
The one true church founded by Christ on Peter and the apostles has always believed and taught the perpetual virginity of Mary by authority of Jesus Christ bound forever! Matt 16:18-19 Matt 18:18
It might go back a ways, but I doubt it has always been taught. It has too much the ring of myth and superstition.