Often, when raising opposition to the doctrine of Mary's perpetual virginity, Christians will insist that we take Scripture at its plain meaning, that we pay attention to the "clear and unambiguous teaching of God's Word." The latter sentiment is a good one; the former is not.
How should we approach Scripture, hermeneutically speaking? This could make for a lengthy topic in and of itself, I suppose, and then one I don't have much time to get into in detail. Most anti-Marian Christians that I have encountered tend to suggest that we employ a firmly literal hermeneutic when interpreting Scripture, and that (as noted above) we take what is written in the Bible at its plain meaning.
The problem with insisting on the use of literal hermeneutics, and with insisting that Scripture be taken at its "plain meaning", is that nobody really does that with any kind of consistency. Let me put that more plainly: no Christian ever takes Scripture at its plain meaning at every opportunity — indeed, many of us are often guilty of a certain hypocrisy when we insist upon just such an approach. More often than not, what we really mean when we insist on taking things at their plain meaning is: "let us use my interpretation of Scripture; let us take it at what I say its meaning is."
That's a rather harsh thing to say, I admit, but let's consider a few examples. There is not a perfect 1:1 correlation between anti-Marianism and a rejection of e.g. Eucharistic doctrine, but most of the Christians I've met who denounce the perpetual virginity of Mary as a flawed Catholic teaching tend to likewise denounce the Eucharist — the real, literal presence of Christ in the bread and wine — as false. And yet, if we take John 6 at its plain meaning, we should have no other choice but to assume that Christ intended for us to receive that which was literally his flesh and blood in our repetition of the Last Supper.
[53] So Jesus said to them, "Truly, truly, I say to you, unless you eat the flesh of the Son of man and drink his blood, you have no life in you;
[54] he who eats my flesh and drinks my blood has eternal life, and I will raise him up at the last day.
[55] For my flesh is food indeed, and my blood is drink indeed.
[56] He who eats my flesh and drinks my blood abides in me, and I in him.
There's really no two ways about that: if our hermeneutic is to always take Scripture at its most obvious meaning, on a passage-by-passage basis, then we should all be Eucharistic Christians. That we are not all Eucharistic Christians means that we do not always take Scripture at its plain meaning; we only do so when it is convenient to the point we are making.
But let's take a few more generalized examples, to better illustrate the point. Consider Jesus' teaching in Matthew 5:
[27]"You have heard that it was said, 'You shall not commit adultery.'
[28] But I say to you that every one who looks at a woman lustfully has already committed adultery with her in his heart.
[29] If your right eye causes you to sin, pluck it out and throw it away; it is better that you lose one of your members than that your whole body be thrown into Hell.
[30] And if your right hand causes you to sin, cut it off and throw it away; it is better that you lose one of your members than that your whole body go into hell.
Taking this passage at its plain meaning, it would be very easy for Christians to justify, say, plucking out the eye of a person caught reading Playboy magazine. Likewise, it would be very easy for Christians to justify lopping off the hands of thieves. And yet, no legal system which as a Judeo-Christian basis to it issues such punishments; moreover, many Christians rightly denounce Islamic countries which do effect such disfiguring punishments upon convicted criminals. Here again, then, we see that we do not always take Scripture at its plain meaning; we only do so out of convenience, when it suits our point to do so.
Similarly, rare indeed is the Christian who takes the Book of Revelation at its plain meaning. The last book of the Bible is full of fantastic imagery and whimsical creatures; it simply cannot be taken at any kind of plain meaning, but is instead widely understood to be a profound metaphor, both for things to come and for things that have already transpired (Revelation is not just an eschatological prophecy; there is strong evidence to suggest that it was also intended as a message of hope to the persecuted churches of the day, using its amazing imagery to describe events happening under the reign of Nero).
In like manner, rare is the Christian who takes James 2:20-24 at its plain meaning — to do so would be to advocate for works-based salvation, which is of course incorrect. Similarly, when Christians look at the Song of Songs, we do not take it at its plain meaning, at least not directly. That book of the Bible mentions God very rarely (you can easily count the instances of His mention in it on one hand, and then probably without using all five digits), and at its most basic meaning is nothing more than page after page of erotic love poetry, a back-and-forth dialogue between two lovers. The most plain meaning at which Christians take that book's contents is as a powerful image of the love that should bless the marital union of man and woman; more often, however, the Song of Songs is understood as a metaphorical image of the intensity of God's love for humanity, which Paul tells us is imaged in the marital union (c.f. Ephesians 5:21-33).
To put it plainly, then, and succinctly: no Christian takes Scripture at its plain meaning at every opportunity — were we all to do so, we would all be Eucharistic in our faith practice, and we would lop the hands off of convicted thieves. Taking the plain meaning of what is written in Scripture can be an important hermeneutical step, to be sure, but it is not a valid hermeneutic on its own. Indeed, Paul confirms this when he instructs us to hold to the traditions by which we were taught (2 Thes 2:15), and to trust in the Church — the pillar and bulwark of truth (1 Timothy 3:15) — to reveal the manifold wisdom of God (Ephesians 3:10) according to those traditions.
This, then, begins to point us in new direction by which we should attempt to understand Scripture and its teachings, and it is this direction which I propose to use in the analysis that follows: let us take Scripture at its intended meaning.
Of course, this is a difficult proposition. Living as we do nearly 2,000 years after the authors of the New Testament, and many additional centuries after the authors of the Old Testament, it is impossible for us to fully know the minds, or the intent, of those who wrote the various books of the Bible. We can only see in a mirror dimly, and can know only in part.
But we are not totally blind, either. The Spirit wove its breath and intent through the whole of Scripture (c.f. 2 Timothy 3:16-17), and one truth cannot contradict another. Therefore, it must be the case that there exists a coherency between the teaching of any one particular part of Scripture and the whole of Scripture.
Jesus actually gives us an example of this principle in action, when he met Cleopas and the other disciple on the road to Emmaus (c.f. Luke 24:13-31). He opened the Scriptures to the two men and, beginning with Moses and all the prophets, interpreted to them in all the Scriptures the things concerning Himself. He didn't just enumerate His own teachings, or his own acts, but tied all of those things into the broader picture of history and prophecy, demonstrating the consistency of the whole of Scripture as it pertained to His own life, death, and resurrection.
That's a lesson for us, in our own attempts to make sense of Scripture and its teachings; it is the model for our own hermeneutics. We should take care to consider each passage and teaching in Scripture in the context of the whole of the Bible. This helps us form an understanding not only of the intent of the authors of any particular part of Scripture (who would, of course, have been mindful of other, pre-existing written texts that eventually became the books of the Bible), but of the intent of the Spirit when and as it inspired them.
So, as we turn now to the formal matter of Mary's perpetual virginity, let us attempt to take Scripture not at its most plain meaning, but at its intended meaning. To do that, we must first journey into the Old Testament, to help us understand a few things about Judaism, the law of Moses, sex and marriage within that paradigm, and celibacy.
Let's begin with the Book of Leviticus.