The only reason for assuming that the Bible says that Jesus was born of a virgin is that Saint Matthew refers to a prophecy about a virgin who will give birth to a son, and that, according to him, came true in Jesus. However, Saint Matthew refers to a text in Isaiah and the original text in Hebrew mentions a young woman and not a virgin.
In several versions of the Christian Bible the prophecy of Isaiah does mention a virgin. What did Isaiah originally refer to: a virgin or a young woman?
Some believe that Saint Matthew misinterpreted the word ‘almah’ because Hebrew was not his mother tongue or because its meaning was no longer the same as in the days of Isaiah. They assume that the Christian Bible changed that word in Isaiah to make it coincide with Saint Matthew. Others are of the opinion that ‘almah’ has always referred to virgin and that those who say the contrary are sceptics.
Both defend their opinion with good arguments. However, it is easy to notice that while some look for the truth, others only defend their belief: they are su sure of it that nothing will make them change their mind.
Both seem to be unaware of the fact that in the days of Jesus the persians were familiar with a Messaih born of a virgin and the Egypcians with a God born of a virgin. Therefore, Saint Matthew may have mistranslated that word to attract the pagans to Christianity. Defending the idea of Jesus’ virgin birth would then imply defending a pagan idea.
https://www.biblestudytools.com/cjb/isaiah/7.html (bible quote: young woman)
https://www.mechon-mamre.org/p/pt/pt1007.htm (bible quote: young woman)
https://theisraelbible.com/bible/isaiah/chapter-7#verse-14 (bible quote: young woman)
https://www.jewishvirtuallibrary.org/yeshayahu-isaiah-chapter-7 (bible quote: virgin)
http://www.hebrew-streams.org/works/hebrew/yeshayah714.html (investigación: unmarried young woman)
http://www.jewishawareness.org/objection-isaiah-714-is-not-a-virgin-birth/ (investigation: pro virgen)
https://www.oneforisrael.org/bible-based-teaching-from-israel/isaiah-714-virgins-birth/ (investigation: pro unmarried young woman)
https://jbqnew.jewishbible.org/assets/Uploads/413/jbq_413_5_almah.pdf (investigation: pro young woman)
https://hermeneutics.stackexchange.com/questions/17042/does-isaiah-714-refer-to-a-virgin (investigation: debate)
https://www.christiancourier.com/articles/461-did-isaiah-prophesy-the-virgin-birth-of-christ; (investigation: pro virgin)
https://en.wikipedia.org/wiki/Isaiah_7:14 (information about the controverse)
https://jewsforjudaism.org/knowledge/articles/isaiah-714-a-virgin-birth/ (investigation: against virgen)
http://www.kjvtoday.com/home/virgin-or-young-woman-in-isaiah-714 (investigation: pro virgen)
https://bibleinterp.arizona.edu/opeds/hag368019 (investigation: pro young woman)
https://www.thegospelcoalition.org/article/jesus-virgin-child-isaiah/ (investigation: pro virgin)
 San Pablo dice de Jesús: “nacido del linaje de David según la carne” (Rm1:3)