Isa 7:14 is a traditional Messianic prooftext. In modern times, however, Matthew's citation is often treated as an embarrassment. Supposedly, Matthew ripped the passage out of context. A few observations:
i) Parthenos isn't Matthew's rendering. Rather, that's the LXX. So that's a pre-Christian Jewish rendering of the passage.
ii) It's often said that if Isaiah wanted to stress the virginity of the woman, he'd use betulah rather than almah. But based on comparative usage, there's no evidence that betulah is more virginal than almah–and possibly less so. (cf. Alec Motyer, E. J. Young, Gordon Wenham, Brevard Childs). Every occurrence of almah arguably refers to a virgin.
(Mind you, there's a difference between what a word means, and what it refers to. But the meaning of a word is established by occurrences of the word. So that's not always so easy to distinguish in practice.)
The real contrast is between almah and ishshah–the customary word for "wife". Some commentators think the text is alluding to the wife of Isaiah or the wife of Ahaz. But in that case we'd expect the text to use ishshah.
In fact, it's striking that the text never identifies the woman. If she was Isaiah's contemporary, why be so reticent?
iii) I think both liberals and conservatives overemphasize the importance of what word is used. Even if Hebrew had a technical term for "virgin," which Isaiah used in this passage, merely using the word "virgin" wouldn't imply parthenogenesis. After all, it's quite possible for a virgin to become pregnant when she has sexual intercourse for the first time. Also, in theory, it would refer to a virgin who will become pregnant subsequent to marriage. The word is consistent with a virginal conception, but doesn't entail that.
iv) In Matthew and Luke, the virgin birth isn't based on a particular word, but on the narrative context, where Mary conceives a child apart from sexual intercourse.
v) In Isaiah, the miraculous connotations of the event aren't confined to how the woman is classified. In addition, this is said to be a "sign" (v10).
In theory, a sign needn't be miraculous. However, the sign is cast in terms that suggest a supernatural event ("Ask a sign of the Lord your God; let it be deep as Sheol or high as heaven"). It might be as spectacular as raising the dead from Sheol. Yahweh gives Ahaz carte blanche.
And that's reinforced by the similar passage in Isa 38:7-8, where God reverses the sundial.
In addition, as R. T. France has noted, the language in Isa 7:14 evokes Gen 17:19, which foretells another miraculous conception.
v) John Walton thinks the sign is the name of the child. Yet the sign is evidentiary. But merely naming a child isn't evidential confirmation of the oracle. That would be self-fulfilling. To function as confirmatory evidence, the sign must be something special.
vi) In addition, signs can be future confirmations ("fulfillment signs"), subsequent to the event in question (e.g. Isa 37:30ff.; 44:28-45:1; Exod 3:12; 1 Kgs 13; 2 Kgs 19). So the passage can be an oracle about the distant future rather than the near future.
vii) In addition to the mysterious woman, you have the even more mysterious child. As scholars like Alec Motyer and Joseph Jensen have documented, the career of this child extends through the events of Isa 11. Projected into a hazy future.
Not coincidentally, having quoted Isa 7 in Mt 2, the evangelist quotes Isa 9 in Mt 4. Matthew perceives the prophetic narrative unity of Isa 7-12. He has a good grasp of narrative theology, and picks up where he left off. Indeed, his discernment is more penetrating than many commentators.
So what we have in Isaiah 7 is a very intriguing prophecy. It would leave the original reader scratching his head. Who is this woman? Who is this child? What is the sign? When will this happen?
The passage raises more questions than it answers. And that's what we'd expect in the case of long-range prophecy, which raises questions that can only be answered centuries later, at the time of fulfillment. The passage is more complex than traditional prootexting suggests. However, the complications reinforce the propriety of Matthew's citation.