Point in case. The word translated as 'virgin' here is 'Almah' in the Hebrew. The word as translated into the Greek, which Matthew used, was 'parthenos' which often meant virgin,but was not exclusively virgin.
The Hebrew term means 'young maiden'.
And, if you read Isaiah on context, Isaiah is talking about his wife, since it says in Isaiah 8:4 "I went to the prophetess and insured she conceived'". Now.. even if she WAS a virgin in 7:14, there is only one way I know of back then a man could 'insure' a young lady conceived.
That doesn't keep her virginity intact.