So, let us have a look at a prophecy where a lot of the discussion back and forth entirely misses the point of the text being discussed. The Christian idea that Jesus was born of a virgin has its roots in the Septuagint translation of the Old Testament, and the evangelists' use of one particular verse.
A lot of the further discussion of this topic basically is philological - does the term almah signify a young woman in general or does it signify a virgin? Arguments in favor of young woman seem way more solid, and even the Greek parthenos seems less clearly to have signified virgins than previously assumed, thus we cannot even be sure that the LXX translators intended the meaning 'virgins'. Further, it seems the texts of the LXX that are accessible to us are all from traditions maintained by Christian scribes, so that weakens the case in favour of virgin a bit as well.
However, there are good reasons even if almah meant virgin to understand the text as not at all being a prophecy about Jesus - and this even if we grant the assumption that God exists, can promise things he fulfills in the future, and also knows the future. These are not assumptions I believe to be true, but let us be magnanimous for a moment!
Therefore the Lord himself shall give you a sign; Behold, a virgin shall conceive, and bear a son, and shall call his name Immanuel.What does any reader in the west when he comes across this verse in isolation think of? Alas, this blinds us to the meaning of the context in which it is found! So strong are our associations, that they blank our minds for a while, our reading comprehension fails.
I have even seen people rejecting the idea that this was written before Jesus was born on the grounds that 'since prophecy really does not exist, Isaiah cannot have predicted an alleged virgin birth, thus it must have been written after the idea of Jesus' virgin birth had taken hold in order to fabricate a prophecy'. This is not a common misunderstanding, but it still shows how relatively reasonable skeptics just fail to read it in context.
Of course, context is a magic word in other ways too - how often do not Christian apologists tell us we must read things in context, meanwhile trying to divert our attention from that very context? In this case, they do not even need to do that, though, since centuries of indoctrination has overruled our reading comprehension.
So, let us look a bit closer at what Isaiah actually says. I will not bother to go through the Hebrew text here, since the English is sufficient to illustrate the point I am making. This is partially based on the exegesis found here.
And it came to pass in the days of Ahaz the son of Jotham, the son of Uzziah, king of Judah, that Rezin the king of Syria, and Pekah the son of Remaliah, king of Israel, went up toward Jerusalem to war against it, but could not prevail against it.So, removing the extra verbiage, Ahaz, king of Judah, learned that Rezin and Pekah, kings of Syria and Israel, were in cahoots to attack him. He was distressed by this. It would seem the author inserts stuff from a later vantage point - "but could not prevail against it" seems to be an indication as to how the narrative will conclude as far as the military campaign goes.
And it was told the house of David, saying, Syria is confederate with Ephraim. And his heart was moved, and the heart of his people, as the trees of the wood are moved with the wind.
Then said the LORD unto Isaiah, Go forth now to meet Ahaz, thou, and Shearjashub thy son, at the end of the conduit of the upper pool in the highway of the fuller's field; And say unto him, Take heed, and be quiet; fear not, neither be fainthearted for the two tails of these smoking firebrands, for the fierce anger of Rezin with Syria, and of the son of Remaliah. Because Syria, Ephraim, and the son of Remaliah, have taken evil counsel against thee, saying, Let us go up against Judah, and vex it, and let us make a breach therein for us, and set a king in the midst of it, even the son of Tabeal: Thus saith the Lord GOD, It shall not stand, neither shall it come to pass.So, God tells Isaiah to talk to King Ahaz, to tell him not to fear. For the record, Ahaz was not a particularly righteous king. Next, God's promise to Ahaz is given in greater detail:
For the head of Syria is Damascus, and the head of Damascus is Rezin; and within threescore and five years shall Ephraim be broken, that it be not a people. And the head of Ephraim is Samaria, and the head of Samaria is Remaliah's son. If ye will not believe, surely ye shall not be established.The promise, quite clearly, is that Syria and Samaria will be crushed. But God has more to say, relating to this very promise about Syria and Samaria.
Moreover the LORD spake again unto Ahaz, saying,So, God wants Ahaz to ask for a sign by which God can affirm this promise - a little miracle to show that he is up to his promise. Ahaz, however, scoffs at this.
Ask thee a sign of the LORD thy God; ask it either in the depth, or in the height above.
But Ahaz said, I will not ask, neither will I tempt the LORD.
And he said, Hear ye now, O house of David; Is it a small thing for you to weary men, but will ye weary my God also? Therefore the Lord himself shall give you a sign; Behold, a virgin shall conceive, and bear a son, and shall call his name Immanuel.So, God himself picks the sign(, and as it turns out decides to renege on what he just told Ahaz). Here, of course, we run into the place where the virgin birth famously is promised. Looking closer at the Hebrew - provided that we grant that Almah is virgin - we find that it is not "a virgin", but "the virgin"; it is also preceded by the somewhat demonstrative הִנֵּה, and thus it seems even more likely the author of this narrative thought of it as the prophet actually pointing out one particular virgin. Of course, there's no oddity about pointing out that someone who currently is a virgin will conceive. She may no longer be a virgin when the conception occurs, obviously, but we may very well identify a person by what qualities they have in the present, regardless of their qualities at another time. However, even then we have good reasons to think it's not 'virgin' that is the intent of the text. Why should anyone think the sign God is giving to Ahaz is the birth of the Messiah? Nothing in the context this far indicates that such an interpretation is valid.
Butter and honey shall he eat, that he may know to refuse the evil, and choose the good.So, the sign by which God will affirm that he has the power to fulfill his promise, is that what he promises will be fulfilled before the boy reaches his teenage years. This is not a very useful sign - it is like "to prove to you that I can repay this loan within ten years, I will repay this loan within ten years". This is really an insult to Ahaz, God is giving a useless sign as a rhetorical device.
For before the child shall know to refuse the evil, and choose the good, the land that thou abhorrest shall be forsaken of both her kings.
The promise is now turned pretty much upside down, with God promising terrible things to befall the kingdom of Judah. So, in case this prophecy really is about virgin births, we must have a virgin birth of the time-keeping boy in king Ahaz time and later on, we have Jesus. That is mighty odd, is it not? However, this means we have a clear idea what the text actually is saying, and the usual understanding that pretty much everyone has if asked what that particular verse means appears incongruous.
Even when reading the text by oneself, it is likely that one reads the bit about the conception and birth as though the prophet temporarily diverts from the affairs of the day to a prophecy centuries into the future and then jumps back to the main topic - as though he was incapable of keeping to a topic.
There still is literature that assumes Isaiah 7:14 is representative of Jewish beliefs about the Messiah, without having looked at any other evidence about the verse in question than its Christian interpretation. This is a pretty important fact regarding how people tend to think of Judaism.
Now, I am not just pointing out what Isaiah actually was saying, I am making a point that this is just one example of - associations that words or phrases have in our minds somehow trumping the actual meaning of the text or utterance, breaking our reading comprehension (or listening comprehension) temporarily. It is a problem lots of people run into, and sometimes makes people come up with ludicrous ways out of the mistaken ideas it causes them - such as the example I mentioned with a skeptic denying that Isaiah could have been written before the birth of Jesus.
In part I suspect this relates to how language works - our brains are pattern-matching algorithms, and a word, a phrase or even a whole poem can trigger associations. Often, these associations are not particularly complicated - but the same word may trigger slightly different associations for each one of us. However, the environment in which most native speakers of English have grown up make Behold, a virgin shall conceive trigger an association with several of Christianity's central tenets. Some other phrases and words may have a comparable power in distorting how we understand a text or utterance, and it is important that we realize that our minds are not immune to this.
 Rabbi Tovia Singer, Dual Prophecy and the Virgin Birth,
Outreach Judaism, http://outreachjudaism.org/dual-prophecy-virgin-birth/
All other quotes from Isaiah 7, King James Version.