Answering purely from Scripture:
One answer is taken from : Mary: Perpetual Virgin? 6 Biblical Arguments | Dave Armstrong, and the rest my own.
There isn't one better than from the article:
Once upon a time, virtually no Christians denied that Mary the mother
of Jesus was perpetually a virgin: including Protestants. Of the early
leaders of that movement, virtually all fully accepted this doctrine:
including Luther, Calvin, Zwingli, Bullinger, and Cranmer. Moreover,
most Protestant exegetes continued to believe it for at least
another 350 years or so. But today (for various reasons) things are
very different, so it’s helpful to revisit the biblical arguments,
since the Bible is the authority all Christians revere in common. A
surprising number can be found: so many that they will require more
than this article to highlight the major ones: even in summary.
1. Mary's answer to the angel.
34 Mary said to the angel, “How can this be, since I am a virgin?”[a]
a. Luke 1:34 Gk I do not know a man
This question does not make any sense at all if Mary and Joseph after marrying, would have had marital relations.
2. Under John’s Protection.
Point 5. in the article The Blessed Virgin Mary is committed to the care of the Apostle John by Jesus from the Cross (John 19:26-27).
Jesus certainly wouldn't have done this if He had brothers (all of
whom would have been younger than He was).
3. The Power of the Most High overshadowing Mary.
35 The angel said to her, “The Holy Spirit will come upon you, and the
power of the Most High will overshadow you; therefore the child to be
born[a] will be holy; he will be called Son of God.
Footnotes: a. Luke 1:35 Other ancient authorities add of you
From Ruth 3:9 (NRSVCE) and from this explanation [t]t is possible that spreading of the garment over a woman was a more primitive form of marriage in Adoption | Jewish Encyclopedia, it is clear why among Catholics Mary is referred to as the Spouse of the Holy Spirit. With this understanding, it is impossible for even the thought to have entered the just man Joseph to approach Mary [as a man goes to his wife] after the Holy Spirit coming upon her. I remember this making a deep impression on me the day a priest explained it to me.
The readers are invited to go through the other arguments in the article.
Marital sex, as wonderful as it is, is a type of the heavenly union that is to come. With Jesus, Mary and Joseph had heaven. It is also easy to understand that celibacy goes back to the Apostles.
Regarding Mary, ALL Christians ought to thank the LORD and be proud that at least one of us never fell under the power of the devil.
You are the glory of Jerusalem ... you are the splendid boast of our people ... God is pleased with what you have wrought. May you be blessed by the LORD Almighty forever and ever. [cf. Jdt 15:9-10].
cf. Protestant arguments for perpetual virginity of Mary.