In those days (end of 1st century, early 2nd century), many Christians, under the influence (or because) of Jewish Christians, avoided to mention the divisive (and controversial) issue of a distinct other God, the pre-existent and eternal "Son of God":
a) Only a few fragments survive from the uncanonical gospel of the Hebrews. This gospel (likely written early 2nd century) enjoyed great favor among Jewish Christians, as reported by Eusebius, 'The History of the Church' (HC):
3, 25 "Some have found a place in the [canonical] list for the 'Gospel of the Hebrews', a book which has a special appeal for those Hebrews who have accepted Christ"
3, 27 "... but nevertheless shared [with the (true) Ebionites] their refusal to acknowledge his pre-existence as God the Word and Wisdom ... using only the 'Gospel of the Hebrews', they treated the rest with scant respect."
From the gospel of the Hebrews itself, we read:
"When the Lord [Jesus, after the baptism] ascended from the water, the whole fount of the Holy Spirit descended and rested upon him, and [the Holy Spirit, NOT the Father] said to him,
"My son, in all the prophets I was waiting for you, that you might come, and that I might rest in you. For you are my rest; and you are my firstborn son,
[of the Holy Spirit (figuratively & happening right after the baptism). No pre-existence here!]
who reigns forever." Jerome, Commentary on Isaiah 11:2
b) Paul in 1Cor 8:6a "yet for us there is but one God, the Father,"
c) Mark 12:29 "The most important one [of the commandments]," answered Jesus, "is this: 'Hear O Israel, the Lord our God, the Lord is one [part of the Jewish Shema] ... '"
d) James 2:19a "You believe there is only one God. Good! ..."
e) Some early Christian writings have Jesus as "Son of God" but do not mention any pre-existence (Ephesians, 2 Peter).
f) Finally, and one step further, some Christian authors even refrained to use the expression "Son of God" (1 Peter, 2 Thessalonians, 1 Timothy, 2 Timothy, Titus).
So a compromise (between a second deity and just a honorary title) was proposed:
Jesus is the Son, not by being the pre-existent Word of God (not acceptable to Jewish Christians), but by having God (or the Holy Spirit) as his biological father. And as reported by Eusebius, that was accepted by the latter Jewish Christians (HC, 3, 27, 27 "They did not deny that the Lord was born of a virgin and the Holy Spirit ..." ).
And the virginity of Mary had become a necessity in order to prevent the thought that Jesus was conceived with a human father!
Note: neither in GMark, GMatthew or GLuke (and 'Acts'), there is a clear mention of the pre-existence of Jesus. What follows can be considered a denial:
Lk 1:32a "He will be great and will be called the Son of the most high"
Lk 1:35b "So the holy one to be born will be called the Son of God"
Ac 17:31 "For he has set a day when he will judge the world with justice by the man he has appointed. He has given proof of this to everyone by raising him from the dead."
Of course, a virgin conception was not new in the Jewish world:
a) Paul himself may have suggested it, about the (promised by God) late (and only) pregnancy of Abraham's wife, resulting in the birth of Isaac, "the son born by the power of the Spirit" (Gal 4:29).
b) According to Philo of Alexandria (20 BCE-50 CE), a popular Hellenistic Jew, scholar, theologian & philosopher:
"Tamar, when she became pregnant of divine seeds, and did not know who it was who had sown them ..." (On the Change of Names, XXIII)
"For when she [Hannah] had become pregnant, having received the divine seed ..." (On the unchangeableness of God, II)
"the angels of God went in unto the daughters of men, and they bore children unto them." (On the unchangeableness of God, I)