Most protestants believe Mary was not a perpetual virgin. The assumption that our blessed Mother had other children comes mainly from Jesus being referred to as her first born.
In references to the words brother and sister of Jesus read in scripture, we need to keep in mind this was a common form of referring and/or greeting one another.
The terminology “first born” was used to designate the heir of the family. In biblical times regardless if a family had one or many children, the title first born was still a customary designation.
When the culture, customs and linguistic forms in the time of Jesus life on earth are not taken into consideration, fallible interpretations are the result, especially when protestants apply their doctrine of sola scriptura, which in itself is greatly flawed and without any biblical foundation.
Therefore based on historical (writings of the early church fathers), principles, traditions and the teaching authority (magisterium) of the Catholic church, it can be affirmed that the perpetual virginity of our blessed Mother is truth.