I'm a Lutheran and we do not believe in the perpetual virginity of Mary. I don't think Luther believed that either.
Luther wrote on the Virginity of Mary:
It is an article of faith that Mary is Mother of the Lord and still a virgin. ... Christ, we believe, came forth from a womb left perfectly intact. (Weimer's The Works of Luther, English translation by Pelikan, Concordia, St. Louis, v. 11, pp. 319-320; v. 6. p. 510.)
You are mistaken. All references are from Luther's own writings.
Christ…was the only Son of Mary, and the Virgin Mary bore no children besides Him… "brothers" really means "cousins" here, for Holy Writ and the Jews always call cousins brothers. (Sermons on John, chapters 1-4, 1537-39).
He, Christ, our Savior, was the real and natural fruit of Mary’s virginal womb…This was without the cooperation of a man, and she remained a virgin after that. (Ibid.)
God says…"Mary’s Son is My only Son." Thus Mary is the Mother of God. (Ibid.).
God did not derive his divinity from Mary; but it does not follow that it is therefore wrong to say that God was born of Mary, that God is Mary’s Son, and that Mary is God’s mother…She is the true mother of God and bearer of God…Mary suckled God, rocked God to sleep, prepared broth and soup for God, etc. For God and man are one person, one Christ, one Son, one Jesus, not two Christs…just as your son is not two sons…even though he has two natures, body and soul, the body from you, the soul from God alone. (On the Councils and the Church, 1539).
It is an article of faith that Mary is Mother of the Lord and still a virgin.... Christ, we believe, cameforth fromawomb left perfectly intact. (Works of Luther, Vol. 11, pp 319-320; Vol. 6,pg 510)
Luther also believed in the Immaculate Conception of the Blessed Virgin Mary.
It is a sweet and pious belief that the infusion of Mary’s soul was effected without original sin; so that in the very infusion of her soul she was also purified from original sin and adorned with God’s gifts, receiving a pure soul infused by God; thus from the first moment she began to live she was free from all sin" (Sermon: "On the Day of the Conception of the Mother of God," 1527).
She is full of grace, proclaimed to be entirely without sin—something exceedingly great. For God’s grace fills her with everything good and makes her devoid of all evil. (Personal {"Little"} Prayer Book, 1522).