…
"Which Church Theologians first proposed that Mary was always a virgin?"
If it wasn’t believed until long after Jesus, what does it matter when Jeromes solution to the Jesus siblings problem came along?
Actually I have done some brief research and answered my own question. Wikipedia again (i know it isn’t infallible, but usually pretty accurate) has anarticle on “perpetual virginity of Mary.”
I quote in part (emphasis mine):
"*A second-century document that paid special attention to Mary’s virginity … this text does not explicitly assert Mary’s perpetual virginity after the birth of Jesus.
There was no full consensus on the doctrine of perpetual virginity within the early Church by the end of the second century, e.g. Tertullian (c.160 – c.225) did not teach the doctrine (although he taught virgin birth), but Irenaeus (c.130 – c.202) taught perpetual virginity, along with other Marian themes.[35] Origen (185-254) was emphatic on the issue of the brothers of Jesus, and stated that he believed them to have been the children of Joseph from a previous marriage.[43] However, wider support for the doctrine began to appear within the next century.[35]
Some writers from 4th century, Helvidius and Eunomius of Cyzicus… after Jesus’ birth, and that James, Joses, Jude, and Simon were the biological sons of Mary and Joseph, …
By the 4th century, the doctrine of perpetual virginity had been well attested.[46] For example, references can be found in the 3rd century writings of Hippolytus of Rome,…4th century works of Athanasius,[48] Epiphanius,[49] Hilary,[50] Didymus,[51] Ambrose,[52] Jerome,[53] and Siricius[54] continued the attestations to perpetual virginity – a trend that gathered pace in the next century.[4][5]*"
So:
It seems to me that the doctrine has a similar history to the Trinity.
It isn’t taught in the Bible, but it slowly evolved over hundreds of years and through many controversies and debates until it was eventually adopted into the Church.

(and afterwards, anyone who questioned it was labelled a heretic no doubt)
With this “truth” decided, and the Bible saying Jesus had siblings, Jerome taught that these siblings are infact cousins and not Mary’s other children at all!

(a better explaination than the idea that Mary was Josephs second wife)
1700 years later JW’s correctly mention Jerome (without satisfactory quotes according to some) as being the first one to teach the cousin theory. (several posters on a discussion website decry JW’s as just swallowing whatever they are told without doing any research!

)
Maybe someone else had the idea before Jerome and he just published it, but regardless, the belief Mary stayed a virgn through her married life hadn’t become doctrine yet, so when the explainations of how that worked came along seems kind of irrelevant to me.
The JW insight book quoted at the begining of this article by Wayntec looks accurate and stands up to research. Good.

Thanks for building my faith in that.