Why did Mary need to remain a virgin during pregnancy according to those who deny the perpetual virginity doctrine? After much editing and clarifying through comments this question seems to have asked and had answered from Catholic perspective why Mary stayed a virgin throughout her pregnancy.

Yet  many Christians do not agree with perpetual virginity doctrine but still  agree that Mary did remain virgin until after giving birth based on Matthew 1:25:


  And knew her not till she had brought forth her firstborn son: and he called his name JESUS (KJV).


What biblical reasons are there for the necessity of Mary refraining from sexual relations with Joseph during the time she was carrying the baby Jesus?
Ever-virgin dogma is the belief that Mary was a virgin during pregnancy, during birth, and after Christ's birth in her marriage to Joseph.

Whether one submits to this Papal dogma or is someone who believes that Christ was born normally or Mary/Joseph had sex and children sometime after His birth, they both, indeed all Christians, believe the prophecy that the virgin would conceive and bear a son.


  Therefore the Lord himself shall give you a sign; Behold, a virgin shall conceive, and bear a son, and shall call his name Immanuel.  Isaiah 7:14


The virgin will CONCEIVE (get pregnant) and the virgin will BEAR (bring forth, travail, deliver).  Two-pronged prophecy:  Conceive and give birth as a virgin.

IOW, even the same question posed for a Catholic answer was given Isaiah 7:14.

So again, the reason why Mary had to remain a virgin during her pregnancy was to fulfill scripture.

PS. This answer really has nothing to do with what came later that the Catholic dogma denies: that is, whether Mary/Joseph consummated their marriage or not (they did), whether Mary took a vow of chastity or not (she didn't), or even whether Jesus was born normally in the normal (human) way (He was).  So, try not to derail the same answer that a Catholic, Orthodox, or Protestant would give.