HERE is a good place to start.
The issue then is not her Virginity after Christ was born, but whether ALL SALVATION is Obtained through Mary as Pope Pius IX said in the UBI PRIMAM of 1854 and the Dogma in 1854:
"For, God has committed to Mary the treasury of all good things, in order that everyone may know that through her are obtained every hope, every grace, and all salvation."
We Protestants maintain that the Lord Jesus Christ IS our ONLY Salvation. His death on the Cross, paid the price of Sin on behalf of folks like us. The Scriptures say there "IS NO SALVATION IN ANY OTHER".
It is the Insertion of the Traditions of Man to give away Salvation to Mary and is a result of the low view of Scripture held.
We Protestants also maintain that Matthew is correct in Verse 1:25 "But he did not have sexual relations with her until her son was born."
Joseph, a Righteous Man, bided his time until the right time to consummate his marriage to his betrothed Mary.