26 And in the sixth month, the angel Gabriel was sent from God into a city of Galilee, called Nazareth,
27 To a virgin espoused to a man whose name was Joseph, of the house of David; and the virgin's name was Mary.
28 And the angel being come in, said unto her: Hail, full of grace, the Lord is with thee: blessed art thou among women.
29 Who having heard, was troubled at his saying, and thought with herself what manner of salutation this should be.
30 And the angel said to her: 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.
34 And Mary said to the angel: How shall this be done, because I know not man? (and ain't NEVER plannin' to!!! Joe don't want me; either)
35 And the angel answering, said to her: The Holy Ghost shall come upon thee, and the power of the most High shall overshadow thee. And therefore also the Holy which shall be born of thee shall be called the Son of God.
A good looking young lady surely needs NO Sugar Daddy to 'guard' her. Guard her from what; someone ELSE marrying her?
Matthew 1:19 Douay-Rheims Bible
Whereupon Joseph her husband, being a just man, and not willing publicly to expose her, was minded to put her away privately. What a GUARDIAN!!
There were many wonder-pregnancies in the OT, as Mary would have known full well, because she was steeped in those stories. Hannah, Sarah, Samson's mother and the rest. And they were to understand that they would conceive by their husbands.
Now, Mary had a husband (Joseph). Why would she be astonished to think she'd get pregnant? Why wouldn't her first thought be that it would be in the ordinary way with her husband?
Now put your thinking cap on and think through this example. Say you're at a wedding shower and all the young women are talking excitedly about the upcoming wedding. They're looking at pictures, commenting in the cuteness of the happy couple, and one of the girls says, "Oh, Olivia, you are going to have such gorgeous children!"
And Olivia says, astonished and perplexed, "But how shall this happen to me, because I know not man?"
???
Huh?
Seriously?
That makes no sense.
Reasonably, once you've eliminated the impossible, whatever remains, however improbable, is the true explanation. Either this gal, betrothed wife of Mr. Right, doesn't know where babies come from (impossible!), or --- this is so improbable, but --- isn't she planning on having sexual relations with him?
As for Joseph divorcing Mary quietly, that doesn't serve as a counter-argument. That doesn't imply anything, except that he's perplexed: is he supposed to be the guardian of her bastard children as well? But he was kind and did not want her exposed to a public shaming. As it was, as soon as the angel provided the explanation about the Divine origin of this Child, he took her into his home as planned, and it was for all the world to assume that the child was Joseph's.