PVM

Since the Catholic dogma of Mary's perpetual virginity (hereafter PVM) recently came up, I'd like to say a few things. 

i) We need to begin with a definition. There's more to the PVM than the claim that Mary never had sexual intercourse. Rather, Rome has a very idiosyncratic definition:

499 The deepening of faith in the virginal motherhood led the Church to confess Mary's real and perpetual virginity even in the act of giving birth to the Son of God made man.¹⁵⁴ In fact, Christ's birth "did not diminish his mother's virginal integrity but sanctified it."¹⁵⁵ And so the liturgy of the Church celebrates Mary as Aeiparthenos, the "Ever-virgin". 

http://www.vatican.va/archive/ccc_css/archive/catechism/p122a3p2.htm

ii) It's unclear what Mary's in partu virginity is supposed to denote. In order to affirm something, it must deny the contrary or contraries. Offhand, I can only think of two logical possibilities:

a) In some miraculous fashion, Jesus passed through the birth canal without rupturing the hymen. Did he momentarily dematerialize, then dematerialize? Did the hymen momentarily dematerialize, then dematerialize? 

b) Jesus was born without passing through the birth canal. Rather, it was a miraculous C-section. Teleportation. "Beam me up, Scotty!" 

iii) I don't have any a priori objection to a miraculous birthing process. However, the stated rationale indicates that there was something improper about the normal birthing process. And that is theologically objectionable.

iv) The onus is not on a Protestant to disprove the PVM. My disbelief is justified by lack of evidence. Indeed, absent evidence, I'd be irrational and irresponsible to believe it. 

Suppose you ask me if I believe in leprechauns. I say, "No." You say, "Prove it!"

Prove what? The onus is not on me to disprove the existence of leprechauns. I don't believe in them for the simple reason that, to my knowledge, there's no credible evidence that they exist. I need no further justification. I don't have to produce evidence against their existence to be warrant my disbelief. Regarding the PVM, the Catholic shoulders the burden of proof. 

v) What would count as evidence for the PVM? Needless to say, there's no available medical evidence. 

At best, it could only be known by divine revelation. Yet Rome can't add to the deposit of faith. So unless it can be proven from public revelation (i.e. the Bible), there's no evidence for the PVM.

There's no point quoting the church fathers. It's not as if the church fathers conducted a pelvic exam of Mary. 

vi) Some Protestants attempt to disprove it. There's nothing necessarily wrong with that effort. Keep in mind, though, that justified disbelief in the PVM doesn't depend on the success of that effort. It is not incumbent on Protestants to disprove a dogma for which there's no evidence in the first place. 

vii) The weakest argument is that Jesus was Mary's "firstborn" son. However, that doesn't necessarily imply subsequent offspring. Given the cultural importance of primogeniture, the birth of a firstborn son was significant in its own right.

viii) A better argument is that the Gospels refer to brothers and sisters of Jesus. Catholics counter that the word can mean cousins. That's possible. But unless there's a presumption that they couldn't be his brothers or sisters (or stepbrothers and stepsisters, to be precise), there's no reason to reach for "cousin."

ix) An even stronger argument involves the "until" clause in Mt 1:25, with its before and after contrast. That's very hard to get around, and there's no reason to evade it unless you have a prior commitment to the PVM–based on what?

x) Finally, as I recently observed, if Mary and Joseph never consummated their marriage, then there's nothing to distinguish their "marriage" from an extension of their betrothal. If Joseph was never the legal stepfather of Jesus, then that in turn delegitimates the theological rationale for the Matthean and Lukan genealogies–both of which trace Christ's ancestry through Joseph:

and Jacob the father of Joseph the husband of Mary, of whom Jesus was born, who is called Christ (Mt 1:16). 

Jesus, when he began his ministry, was about thirty years of age, being the son (as was supposed) of Joseph, the son of Heli (Lk 3:23).