Here’s how LDS came to believe this:
Isaiah 7:14 Therefore the Lord himself shall give you a sign; Behold, a virgin shall conceive
, and bear a son, and shall call his name Immanuel.
1 Nephi 11:18 And he said unto me: Behold, the virgin whom thou seest is the mother of the Son of God, after the manner of the flesh.
Alma 7:10 And behold, he shall be born of Mary, at Jerusalem which is the land of our forefathers, she being a virgin, a precious and chosen vessel, who shall be overshadowed and conceive by the power of the Holy Ghost, and bring forth a son, yea, even the Son of God.
Luke 1:31-35
*31 And, behold, thou shalt conceive in thy womb, and bring forth a son, and shalt call his name Jesus.
32 He shall be great, and shall be called the Son of the Highest: and the Lord God shall give unto him the throne of his father David:
33 And he shall reign over the house of Jacob for ever; and of his kingdom there shall be no end.
34 **Then said Mary unto the angel,How shall this be, seeing I know not a man?
35 And the angel answered and said unto her, The Holy Ghost shall come upon thee, and the power of the Highest shall overshadow thee: therefore also that holy thing which shall be born of thee shall be called the Son of God.**
LDS believe the same. I hope this helps.*
Mary was a betrothed virgin, and obviously intended to stay that way, otherwise why would she say what she did to the angel, “how shall this be, since I have no relations with a man?” Surely, Mary was quite aware of how children are physically/sexually conceived, don’t you think? The previous verses to this one help further to explain Mary’s question:
28
And coming to her, he said, “Hail, favored one! The Lord is with you.”
29
But she was greatly troubled at what was said and pondered what sort of greeting this might be.
30
Then the angel said to her, “Do not be afraid, Mary, for you have found favor with God.
31
Behold, you will conceive in your womb and bear a son, and you shall name him Jesus
32
He will be great and will be called Son of the Most High,* and the Lord God will give him the throne of David his father,
33
and he will rule over the house of Jacob forever, and of his kingdom there will be no end.”
34
But Mary said to the angel, “How can this be, since I have no relations with a man?”*
35
And the angel said to her in reply, “The Holy Spirit will come upon you, and the power of the Most High will overshadow you. Therefore the child to be born will be called Holy, the Son of God.
Now, if Mary had planned to have relations with Joseph, then of course she could have conceived a child immediately, and what the angel said would have made total sense to her. Evidently, this is not what Mary had planned though. Also, the angel said Mary would conceive
a son, not a daughter. Who has control over what sex your conceived child will be ahead of time, except God, right? Already we see that there is something unique going on here, not normal sexual intercourse between a man and a woman, correct?
Jesus, though He was conceived in Mary, and was fully human, also was divine in nature. The angel said how this would happen in verse 35, the Holy Spirit would come upon her, and the power of the Most High
would overshadow her and the child would be born Holy, the Son of God. The language used here, “come upon her”, and “overshadow her”, denotes a particular way in which she would conceive by the Most High God, not at all like a male and female human would conceive a child, in a physical contact/sexual way.
You don’t think God could make Mary conceive any other way than coming down and actually laying with her?