To understand the virginity thing one has to less look at the life time history of Jesus of Nazareth but rather look at medieval Church history. The gospels of the Bible barely tell us anything about the historical Mary (even so one has to keep in mind that the earliest gospel was written about a century after the death of Jesus and the details of the gospels often don't agree with one another, e.g. Bethlehem and Nazareth are both given as the birth place of Jesus and Nazareth seems to be more historical probable one).

What we know about the woman Mary (or rather Miriam) is that she was married to a man named Joseph, her first born child was Jesus and she had several other children after him. In the gospel of Matthew the legendary lineage of Jesus is given, beginning with Abraham and David and ending with Joseph as 'husband of Mary, mother of Jesus' instead of just saying 'father of Jesus'. Since other female figures and their role in human reproduction aren't considered worth mentioning the implication ist that Joseph was not considered the biological father of Jesus but merely acting in loco parentis (and fulfilling the genealogical role of tying Jesus back to Abraham and David, if not in flesh then at least in spirit). In the gospel of Luke there is the description of how Mary is visited by an angel, telling her that she will give birth to the Messiah (= "saviour of the jews"). Mary questions the possibilty of this since she doesn't know any men ('knowing' may probably refer to 'having intercourse with' in this context, however that is not as unquestionable as it is often presented with this phrase, earlier in the Bible in Genesis it is told that Adam 'knows' every animal in the Garden of Eden, so...) The angel answers Mary that Holy Ghost will come upon her. This is interpreted as meaning that the Holy Ghost will literally impregnate her instead of her soon to be husband but this again is up for interpretation. Fact is that the historical Jesus never referred to himself as the son of God as far as we know from the source material.

Now how does the Virginity come into play? The concept of Mary as a virgin does not actually come from the New Testament but refers back to the Old Testament. We have seen time and time again that the figure of Jesus gets tied in with prophecies and figure of the older, Hebrew Bible to legitimize his claim as Messiah. Not by Jesus himself, mind you, but by later interpretators. E.g. the lineage drawn from Abraham and David, covering every important figure of Hebrew history by whatever thin thread even if it just ends with 'Joseph, not biological father'. Two gospels give Bethlehem as Jesus' place of birth because of a prophecy that the Messiah will be born in this town. If you want to prove that your figure is the Messiah and everyone knows this prophecy then your figure does have to be born in Bethlehem in your text other historical sources notwithstanding. With Mary's virginity it is the same thing. Mary is considered a virgin not because of anything in the New Testament or Jesus' words (keep in mind, on several instances Jesus' many siblings are mentionend) but because of the prophecy in Jesaja 7, 'the Lord will give you a sign, a virgin will get pregnant and give birth to a son and she will name him Emmanuel'. Well, first problem first, Jesus is not named Emmanuel, so already this prophecy appears to be on shaky ground. Second problem, the "virgin". This passage is part of the Hebrew Bible/the Old Testament and as such most commonly surving in to later times as the Septuaginta, the Greek translation of the original Hebrew and Aramaic texts. The Septuaginta uses the translation "parthenon" which in turn got translated into Latin as "virgo"- virgin. Early Christians tried, once again, tying back Jesus to the prophecy of Jesaja to legitimize the claim that Jesus is the long ago foretold sign of God and thus the legitimate Messiah. So Mary became interpreted as the "parthenon" in this prophecy. And so it became Church dogma that Mary was a virgin. Because if Mary was not a virgin then Jesus was not The One foretold by the Old Testament, and then everything Christins believed in might also be in doubt. You see how it goes.

However, historians deciphering the original texts of the Septuaginta have discovered by now that instead of "parthenon/virgin" the original text uses a word more commonly to be translated as "young woman". So Mary never calls herself a virgin and especially not after having giving birth the first time, Jesus never describes his mother as a virgin, and God never send the prophecy to impregnate a virginal woman (or impregnate anyone for that matter).

Why we still have the dogma of Virgin Mary and why it is so fundamental to Christianity I can cover in a later post if you are interested (right now I don't have the time to elaborate further unfortunately).

Sources: David Chidester (2000): "Christianity. A Global History", HarperOne https://www.welt.de/kultur/article3100214/Uebersetzungsfehler-machte-Maria-zur-Jungfrau.html