Luke 1:34

Then Mary said to the angel, “How can this be, since I do not know a man?”
Luke 1:34 Modern English Version (MEV)
 
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Then said Mary to the angel, how shall this be,.... This she said not as doubting the truth of what was said; for she required no sign, as Zacharias did; nor is she charged with, and blamed for unbelief, as he was; yea, it is expressly said, Luk_1:45 that she believed: nor was this a curious question, as whether she should have this son by a man in a married state, or in her present virgin state; for she clearly understood the angel to mean the latter; and therefore her words express her admiration at it, and also her desire to be informed of the manner how it should be: as to the matter of fact, she did not dispute it, but wanted to be resolved by what means it would be brought about: she knew, by prophecy, that the Messiah was to be born of a virgin, and she perceived, by the angel's declaration, that she was that virgin, but could not imagine in what way this amazing thing should be effected; and therefore proposes this question for the following reason,
 
seeing I know not a man? "A husband", as the Arabic version renders it; not Joseph, nor any other man; for though she was espoused to Joseph, yet he had not taken her to wife; nor were they, as yet; come together; and before they did, she was found with child of the Holy Ghost, Mat_1:18 she was a pure virgin, untouched by man. The words are an "euphemism", or a modest way of expressing carnal copulation; see Gen_4:1.
 
John Gill’s Exposition of the Bible
 
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There have been efforts over the years to attempt to explain away the virgin birth by insisting the proper translation for “virgin” is young woman, or something along those lines. While I personally do not have the knowledge, training or expertise to argue one way or the other, I would point out that Mary’s response to the announcement of the angel seems pretty clear.