Why was it neccessary to think of Jesus in terms of a virgin conception? Were people (jewish) aware of the legends about Alexander, and therefore, the first Christians started to describe Jesus in terms that elevated him about ordinary men?
Being Jewish myself, I cannot say that I know anything from that era that suggests that the first Christians were influenced directly from the legends of Alexander to state that Jesus was born of a virgin. But were those Jews aware of the legends surrounding Alexander's birth, the answer would have to be yes.
But to jump to the conclusion that just because two things appear to be alike that one must be borrowing from the other is not right to do without conclusive evidence. What I do know is that the Jews of the time did not know that women had an ovum that needed to be fertilized by the semen of men.
Instead, the Jews believed that women had all the product within themselves necessary to bring forth humans. Psalm 139:13 speaks of the "fluids" within the womb that God "knits" together, speaking of them in Hebrew as if they are like curdled milk that can be molded. The act of adding the "seed" of man was the generative force or principle that would activate the natural process of forming the new human body.
By the time that the Gospels were composed, there were already Greek disciples in every congregation or area in which each of the gospel books were composed: Mark was likely written in Rome, Matthew was written in Antioch, Syria, while Luke wass a Gentile writing in Hellenistic Greek for a Gentile audience, and John was composed in Ephesus or Antioch. None of the gospels originated from the Jerusalem Church or were composed by the Jerusalem bishop, James the Less.
This means that Greek thought was definitely going to be influencing the gospels, not to mention that they were composed in the Greek language. Remember, Jesus and his apostles spoke and taught in Aramaic and Hebrew. All his speeches would be translated into Greek for these gospel narratives, so it is obvious some things would be lost in translation.
The Jewish view that the male "seed" would "kickstart" the process of generating the human form in the womb might work in the case of Jesus--if the starter were God, except for the fact that there is no Jewish prophecy that states that the Jewish Messiah is to be born of a virgin.
The idea in Matthew 1:23 is the only place where any of the Gospels mentions that Jesus was born of a virgin as fulfilling prophecy. And even there, the writer makes no note of this as a marvel. He mentions it only as if in passing. He concentrates, instead on the fact that Jesus is "Emmanuel" or "God with us." Isaiah 7:14, which is being quoted here, is not a text about the Messiah.
Commentators agree that the author is concentrating not on the fact that Jesus is born of a "virgin," but that Jesus is fulfilling the role of "Emmanuel," that Matthew is calling Jesus "God incarnate" or "God with us." The reason is that Matthew is quoting a verse from Isaiah that by chance renders a word that reads as "virgin" in Greek but is not the point of Matthew's use of the verse. In the past too many have focused on the word "virgin" when the author was making the point of the word "Emmanuel" and Jesus being the "God" who is now "with" his people in the person of Jesus.
Regardless if the authors were inspired by Hellenistic thought or not, the idea was, as you stated, to describe Jesus in terms that elevated him above ordinary human beings. I am likely to lean toward saying that Hellenistic thought had to be influencing the Gospel movement because these ideas were not part of the Jewish hope regarding the Messiah.
The Jewish Messiah, in the Second Temple era, had to be the son of a man who was of the tribe of Judah, of the house of David. If he was born of a virgin and was the son of a deity, he could not be the Jewish Messiah.
The tales of Jesus' sonship came some 30 years after Jesus' death. The details of the Jewish Messiah had to be known during his lifetime, meaning they had to be highly public as he would had to be a public figure. Jesus of Nazareth, especially in the Gospel of Mark, constantly told people to hide the fact that they knew he was the Messiah or that they saw him work miracles that proved he was the Messiah. These "miracles" had to be public, like the works of Moses, to be considered worthwhile.
The story of Jesus' birth begins like a legend and reads like a legend because, with all due respect, it is a legend. While I believe in the historicity of Jesus, and I do believe he was a rabbi and sage when he grew up, and he was later crucified by the Romans and declared Messiah by his believers, he never rose from the dead. The stories about him are powerful, awe-inspiring, encourage faith in Jesus--but they are not real or true.
Like the many Hebrew myths found in the Jewish Bible, the Christian Scriptures repeated much of the same elements in their stories to draw believers in their movements. Their Bible is also filled with the same contradictions and errors that plague the Hebrew Scriptures too. Christians may not like to hear this, but unlike the Jews they are often not exposed to critical thinking and analytical study and practical application of their tradition.
It wasn't the Jews who wrote the legends of Jesus. It was the Christans, the amalgamated group that had to make something new up after their leader was crucified. Much like the failed 1914 date when the world didn't end for the Rusellites, the Christians had to make something of their leader being put to death. The new spin? They made him into a God who was born of a virgin and rose from the dead after being put to death as a criminal.