Morningstar,
It seems that the problem lies in dual notions of what "virginity" means. The premise that Muller and Bux are working off of is that "virgin" means never having engaged in relations with man.
Those who want to prove Muller/ Bux are heretical are claiming that virginity ALSO means the integrity of the biological parts. I think the proposition that this additional meaning of virginity is NECESSARILY demanded by Catholic Dogma is what these people have to prove.
Morningstar, your quote seems to prove the point of Muller and Bux, or at least is consistent with their interpretation. Let me explain.
(2) Perpetual Virginity
The expression perpetual virginity, ever-virgin, or simply "Mary the Virgin" refers primarily to the conception and birth of Jesus. From the first formulations of faith, especially in baptismal formulas or professions of faith, the Church professed that Jesus Christ was conceived without human seed by the power of the Holy Spirit only. Here lies the decisive meaning of expressions such as "conceived in the womb of the Virgin Mary," "Mary's virginal conception," or "virgin birth."
Muller and Bux are also saying that that "perpetual virginity" means conceived in the womb of the Virgin Mary," "Mary's virginal conception," or "virgin birth." To them "virgin birth" simply means that no relations with man occurred to cause the birth.
The early baptismal formula (since the 3rd century) state Mary's virginity without further explaining it, but there is no doubt about its physical meaning. Later statements are more explicit. Mary conceived "without any detriment to her virginity, which remained inviolate even after his birth" (Council of the Lateran, 649).
Again, Muller and Bux can agree with this statement as the fact that Mary had no relations with man (virginity) did indeed remain inviolate after Christ's birth. Mary did not have any relations with man during her entire life. Therefore, after Christ's birth she was as virgin as before.
Although never explicated in detail, the Catholic Church holds as dogma that Mary was and is Virgin before, in and after Christ's birth.
Again, Muller and Bux can agree with this. Mary never once knew man and thus was perpetually virgin.
Neccessarily adding physical integrity of the biology in question here to the very definition of "virginity" causes problems. Consider:
1.) A man and woman can biologically have relations in some cases without violating the physical integrity of the woman. Without getting too graphic here, there are cases when stretching (not breaking) occurs. Is the woman in this case, still a virgin?
2.) Let's say a woman has absolutely no relations with man, but due to some sort of injury to the area, the nature of which is completely unrelated to relations or any sin, loses the physical biological integrity of that area. Is she no longer a virgin?
3.) Let's say a woman has had absolutely no relations with man. She is articificially inseminated (which we all agree is a sin) and a baby is conceived and she bears the baby, violating her physical integrity. Is this woman still not technically a virgin (yet a sinner due to the artificial insemination), since she has had no relations with man at any time?
Is it not possible that in previous times, biological integrity was "proof" of virginity and thus the two were intimately related? Thus, perhaps in previous times it would have been dishonorable in society to have one's biological integrity not intact even if there were no sin committed. Thus, there may have been an impetus for certain theologians of the age to develop the notion that Our Lady was saved from any violation of bodily integrity as well? And there is nothing wrong with this belief, certainly, for who really knows for certain? I can't imagine, God forbid, there were ANY witnesses to attest one way or the other, even St. Joseph himself, obviously, since Our Lady was perfectly modest. Therefore, we are going on speculation are we not? The only way we can say for certain is if the theology of what else we definitively know, logically compels us to believe this was the case.
Thus, again, it is the burden of those who cite Muller and Bux with heresy, to prove that the definition of "virginity" in this case, necessarily includes the notion of physical bodily integrity of that region. Otherwise, it clearly can be read to mean "no relations with man" and thus Muller and Bux's views cannot be said to be heretical.