Elsewhere I have documented how Luther (along with virtually all of the Protestant founders) firmly believed in the perpetual virginity of Mary (see separate papers regarding the same belief of Calvin, Bullinger, and Turretin), and that the in partu virginity of Mary is a required Catholic dogma. I stated in the latter paper:
"Ever-Virgin" means conception while remaining a virgin (Virgin Birth), virginity during childbirth, and perpetual virginity after the birth of Jesus (no siblings of Jesus or sexual activity).
The Church has interpreted Mary's virginity during the birth (in partu) as an inviolability of the hymen; in other words, it was a physically miraculous birth rather than a natural one.
This is what Luther also believed. Here is the confirmation and primary source documentation from scholars:
Luther goes further than this to express a belief in the continuation of Mary's virginity beyond Jesus' birth; although he would not want to praise Mary's perpetual virginity as a model for a life of avowed celibacy, he did want to support the belief in Mary's virginity ante partum, in partu, and post partem expressed by the Council of Chalcedon in 451. 39
[39 For Luther's support of Mary's virginity in partu see WA 17II:457; 7:549; 11:320; for post partem see WA 11:320; 49:174 and 182; 51:176; 54:207.]
(in Timothy J. Wengert, editor, The Pastoral Luther: Essays on Martin Luther's Practical Theology, Grand Rapids, Michigan: Wm. B. Eerdmans Publishing, 2009, chapter by Beth Kreitzer: "Luther regarding the Virgin Mary," pp. 233-250; quotation from p. 246; her chapter appeared originally in Lutheran Quarterly 17 , 249-266)
Luther also asserted in later sermons that Mary was 'virgo ante partum, in partu et post partum': WA, vol. 49, pp. 174, 182; vol. 54, p. 207.
(The Cult of the Virgin Mary in Early Modern Germany: Protestant and Catholic Piety, 1500-1648; Cambridge University Press: 2007, p. 57)
Tim S. Perry
'Freedom of a Christian' (1520) cites her willingness to undergo purification in spite of her in partu virginity to exemplify the obedience of faith (LW 31:368), . . .
[Luther affirmed Mary's perpetual virginity throughout his life, commenting on it as late as 1545. See "Prefaces to the Old Testament" (LW 35:275). ]
(Mary for Evangelicals: Toward an Understanding of the Mother of our Lord, Downers Grove, Illinois: InterVarsity Press, 2006, p. 214)
David C. Steinmetz
In fact Luther had no quarrel with the old Christian tradition that affirmed the perpetual virginity of Mary and embraced the teaching that Mary was a virgin ante partum, in partu, and post partum.
("Luther and the Blessing of Judah," pp. 159 ff.; citation on p. 159; in International Congress for Luther Research [10 : 2002 : København]: Luther nach 1530, Theologie, Kirche und Politik [Vandenhoeck & Ruprecht, 2005] )