I've found some very interesting quotes of Martin Luther:
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.
Luther's Works, eds. Jaroslav Pelikan (vols. 1-30) & Helmut T. Lehmann (vols. 31-55), St. Louis: Concordia Pub. House (vols. 1-30); Philadelphia: Fortress Press (vols. 31-55), 1955, v.22:23 / Sermons on John, chaps. 1-4 (1539)
Christ . . . was the only Son of Mary, and the Virgin Mary bore no children besides Him . . . I am inclined to agree with those who declare that 'brothers' really mean 'cousins' here, for Holy Writ and the Jews always call cousins brothers.
Pelikan, ibid., v.22:214-15 / Sermons on John, chaps. 1-4 (1539)
A new lie about me is being circulated. I am supposed to have preached and written that Mary, the mother of God, was not a virgin either before or after the birth of Christ . . .
Pelikan, ibid.,v.45:199 / That Jesus Christ was Born a Jew (1523)
However I found no other explanation for this issue except that both Luther and Zwingli were raised in the teachings of catholic church.
As for the authority of the doctrine I think it relies on the early teachings of the church, as it is reflected in the teachings of the first fathers:
- Athanasius (Alexandria, 293-373);
- Epiphanius (Palestine, 315?-403);
- Jerome (Stridon, present day Yugoslavia, 345?-419);
- Augustine(Numidia, now Algeria, 354-430);
- Cyril (Alexandria, 376-444);
For example, the title Mary Ever-Virgin is found in is found in Athanasius'. Orat. ii. § 70. (Orations against the Arians Book II section 70) "Let those who deny that the Son is from the Father by nature and proper to His substance, deny also that He took true human flesh of Mary Ever-Virgin." You can read more in newmann reader under the title Mary Ever-Virgin.