The word – עַלְמָה– ʿal’mah refers to a human female between the ages of twelve years (legal age of marriage) through nineteen years. At the age of twenty years she becomes – אִשָּׁה– ʾishah regardless of whether or not she is married or has children.
There is no Mitz’vah that prohibits premarital sex; nor is there any Mitz’vah that requires virginity. In fact the only person required by Torah Law to marry a virgin is the High Priest. There is no reason to assume that all human females 12 – 19 years of age are virgins. At the time that I was in that age group, 1969 – 1976, which was shortly after moving to the United States from Yis’raél, there were very few girls in that age group who were still virgins, and of those who were, many still are.
However setting all of that aside, all anyone has to do is continue reading the passage. The identity of the young woman who is giving birth in Y’sha’yahu 7:14 is revealed in Y’sha’yahu 8:3; an unnamed prophetess with whom Y’sha’yahu had sexual intercourse. Therefore she was obviously not a virgin. Was she his wife? It doesn’t say, nor does it matter.
As I said before there is no Mitz’vah prohibiting premarital sex; nor in fact is there any Mitz’vah prohibiting extramarital sex between a married man and a single female. Although polygamy and polygyny are illegal in modern Yis’raél, both are indeed allowed by Biblical Law.