I had a seminary course in Augustine, and in spite of his brilliance and fantastic contributions (probably in the top 5 of intellects in the history of the Church...along with St. Paul and Aquinus) he also carted a lot of his neo-Platonism into the Church...which has been nursing it ever since.
The whole concept that Jesus birth MUST itself have miraculously not broken his mother's hymen--keeping her a perpetual virgin afterwards (also an extra-biblical idea...why?) seems to me just points to some men who had serious issues with normal human sexuality--particularly in women.
In fact is in Augustine's day, one raging issue was whether in order to be a true Christian at all celibacy was required. The other side of the issue said it was a silly requirment--that celibate Christians were no more holy than married ones...(actually arguing from the same bible verses used by Protestants on this issue today).
St. Augustine split the difference, saying that while celibates led a higher life, it was perfectly fine for baptized Christians to marry and have families (good thing--or had the extremist side won, Christianity may have died out in a generation or two). The division though of saying celibate Christians have a greater degree of spirituality led eventually to the requirement in all orders of priests for celibacy. More than a practical requirement for some missions (which seems perfectly reasonable to me--and follows the reasoning of the Apostle Paul) Augustine made it a spiritual issue, something scripture never hints at.
Augustine also went on to speculate that the reason orginal sin was passed on to ones children was that the sexual act, even in married people, invariably involved "concupiscence" ("lust") and hence conception occurred during sin....
As to why it was desirable that Mary's virginity be preserved AFTER Jesus' birth, I really do not understand; but I think it has more to do with Pagan notions of spirituality (untainted by the body...) than any serious true tradition. So strong is this idea that virtually all the first generation Protestant reformers held to it--in spite of silence (or even contradiction) by the bible.
Reading from 1600 years later vantage point, it seems clear to me that many of the Fathers (along with tens of thousands of regular Christians of that day) brought with them, in addition to their brilliance and education, a legacy of pagan dualist philosophy, which had serious problems with the God-created goodness of the physical body.
Christianity is still struggling with that legacy today--in both Roman and Protestant circles.
In any event, most protestants make the same mistake thinking that each and every idea by a particular Church Father has become part of the Church Tradition. That is not accurate. The Church Herself, guided by the Holy Spirit decides what part/parts of the Theological Speculations/Ideas/Systems she will adopt as Doctrine and GREAT men like St. Augustine not only knew that, they taught that. That is, St. Augustine himself wrote that he submitted ALL of his Theologising to the Catholic Church for judgment.