It's not that straight forward:

• Isaiah 7:14 uses the Hebrew word "עלמה" (translation: "maiden, young woman or virgin") when (allegedly) prophesying Jesus.
• Matthew 1:23 quotes Isaiah 7:14, but since the New Testament is in Greek, it uses the word "παρθένος" (translation: "virgin").

This is what is perceived to be the mistranslation. It could be seen as that, or it could be seen as being more specific. Either way, Mary was most definitely a virgin according to the Bible due to Luke 1:26-35:

In the sixth month, the angel Gabriel was sent by God to a town in Galilee called Nazareth, to a virgin engaged to a man named Joseph, of the house of David. The virgin's name was Mary. And [the angel] came to her and said, "Rejoice, favored woman! The Lord is with you." But she was deeply troubled by this statement, wondering what kind of greeting this could be. Then the angel told her:

Do not be afraid, Mary, for you have found favor with God.

Now listen:

You will conceive and give birth to a son, and you will call His name JESUS.

He will be great and will be called the Son of the Most High, and the Lord God will give Him the throne of His father David.

He will reign over the house of Jacob forever, and His kingdom will have no end.

Mary asked the angel, "How can this be, since I have not been intimate with a man?"

The angel replied to her:

The Holy Spirit will come upon you, and the power of the Most High will overshadow you.

Therefore the holy One to be born will be called the Son of God.