Beyond the Scriptural data which clearly teaches the virginity of Mary at the time of Jesus' conception, there is the issue of the extra scriptural tradition of her perpetual virginity (semper virgo). As it is well known, the perpetual virginity of Mary was taught widely in the early Church, some claim as early as St. Ireneaus in the late second century. The doctrine was also supported by Luther, Zwingli, Calvin,and the later Lutheran Scholastics. The Protestant Reformers and Scholastics mainly drew their arguments in favor of Mary's perpetual virginity from St. Jerome's work, Against Helvidius. This teaching continues to be upheld by the Roman Catholic Church to this day.
The difficulty with this doctrine, is of course, that Jesus is clearly stated to have brothers and sisters in the Gospels (Mt 12:46, 13:55-56, Mk 3:31-34 Lk 8:19-21, Jn 2:12). Other objections have been raised by verses such as "But he [Joseph] had no union with her until she gave birth to a son" (Mt. 1:25, Emphasis added). In other words, this seems to suggest that Joseph did have sexual intercourse with Mary after Christ was born. To these objections, Jerome argued that within their biblical idiom, the words "brothers" and "sisters" could also mean cousins or even just countrymen. It might also be argued that they were children from a previous marriage of Joseph. In that Joseph does not appear in stories concerning Jesus' later ministry (unlike Mary) it is possible that Joseph was considerably older than his wife and could have been married to another at an earlier date. Nevertheless, this is amounts to speculation. Regarding the "not until" of Matthew 1:25, Jerome claimed that this was a mere turn of phrase, similar to "before he repented, he was cut off by death." Of course, the person in question never did repent and consequently saying "before" does not mean that they eventually repented.
Jerome's main goal here was not simply to vindicate a tradition of the early Church. Helvidius had claimed that virginity is no better than matrimony and children in the eyes of God. To prove this, he had stated that because Mary, one "blessed among women" had occupied both, that both must be equally good. To counter this claim and laud the superiority of virginity, Jerome did his best to vindicate the tradition.
Although Luther and the majority of the Lutheran tradition prior to the Enlightenment held to the doctrine of the perpetual virginity, Lutheran theology must be wary of accepting this idea for several reasons. The first difficulty is the lack of Scriptural data. Although we do not have the space here to engage in a thorough exegesis of every passage that Jerome cites, let us posit for the sake of argument that all of his exegesis is essentially correct. Even if we do this, his arguments do not positively vindicate the tradition on the basis of Scripture. Jerome's argumentation style is rather ad hoc. What he suggests is that the word usage of Scripture provides wiggle room. If one accepts that this wiggle room exists and therefore the possibility that the texts can be read in such a way as not to exclude perpetual virginity, the exegete can take the next step and read the text in light of the extra biblical tradition. By these means, semper virgo becomes exegetically plausible. If one was not motivated by imperative of the extra biblical tradition and one was simply left to make a decision on the question purely on the basis of Scripture alone, it is difficult to see how one would come to conclusion that Mary always remained a virgin. Even Jesus' turning his mother over to the disciple John (Jn 19:26-7) is far too ambiguous to make this a positive of suggestion. Again, read in isolation, there are any number of alternative reasons why Mary's natural sons might be incapable of taking care of her. In essence, the difficulty is that the exegetical method here employed allows a non-biblical tradition to not merely shed light on a passage in Scripture, but be the determining factor in the interpretation the Scriptures. This is not acceptable in light of the Reformation principle of scriptura sui interpres.
The second point is that Lutheran theology cannot accept the idea that the state of perpetual virginity is inherently superior to married vocation. Jesus and Paul certainly do praise virginity for those who can accept it (Mt 19:3-12, 1Cor 7:8-9, 27, 32-35, 38), but this does not negate other vocations (1 Cor 7:7). God placed Mary in a married vocation and it would have been in violation of God's commandment at the beginning of creation (Gen. 1:28) if she did not engage in sexual intercourse with her husband.
This brings us to one of the most puzzling aspects of the doctrine of the perpetual virginity. Namely, if Mary decided to remain a virgin perpetually, were did she get the idea? The idea that she did so of her own accord without a Word from God suggests a sort of Enthusiasm on her part. This would have been rather inconsistent with her faithful harkening to the Word of God spoken to her by the angel (Lk 1). One might also ask as to what purpose always remaining a virgin within marriage might serve. It is easy to see why Jerome and the Latin tradition in general would think of virginity as superior. If it meant a greater degree of self-denial, it could function as an act of supererogation. In contrast to this, Evangelical Lutheran dogmatics posits on the basis of Scripture that no vocation can be considered meritorious (Lk 17:7, 1 Cor 7). All are justified by faith and all serve an equal and important function in the one body (Rom 12, 1 Cor 12).