Christian Bibles render Isaiah 7:14 as, "Therefore the Lord Himself will give you a sign: Behold a virgin will be with child and bear a son, and she will call his name Immanuel." Isn't a virgin being with child an outstanding "sign"?
On the contrary! It is no "sign" at all. Mary's alleged virginity was not something that could be openly displayed as a proof that something out of the ordinary happened. People would only scoff at such a claim no matter how virtuous the girl was thought to be. A "sign" must be something all can see. An alleged "sign" that would require a gynecological examination of Mary is simply not what is meant by a "sign" in the Jewish Scriptures.