Interesting theory. The angel says (I quote in full)
"[30] ...Fear not, Mary, for thou hast found grace with God. [31] Behold thou shalt conceive in thy womb, and shalt bring forth a son; and thou shalt call his name Jesus. [32] He shall be great, and shall be called the Son of the most High; and the Lord God shall give unto him the throne of David his father; and he shall reign in the house of Jacob for ever. [33] And of his kingdom there shall be no end.".
Acciording to you, Mary listens to that and thinks, "yikes, premarital sex!"
Seriously? Where is that "immediacy", by the way?
What are you always yelling at us to do? "READ the rest of the verse!". It continues:
Luke 1:35-41
"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. And, behold, thy cousin Elisabeth, she hath also conceived a son in her old age: and this is the sixth month with her, who was called barren. For with God nothing shall be impossible. And Mary said, Behold the handmaid of the Lord; be it unto me according to thy word. And the angel departed from her. And Mary arose in those days, and went into the hill country with haste, into a city of Juda; And entered into the house of Zacharias, and saluted Elisabeth. And it came to pass, that, when Elisabeth heard the salutation of Mary, the babe leaped in her womb; and Elisabeth was filled with the Holy Ghost:"
So, as you can see, Gabriel told Mary about Elisabeth, who was in her sixth month of pregnancy and she hurried to visit her. Mary was engaged to be a normal married Jewish woman and she knew she could not be pregnant until she and Joseph were over the betrothal period. That is where I see "immediacy". See?