Greek has no undefinite article. It has a definite article, absent in this case: "epei andra ou ginosko". Your case would be served if Luke wrote "epei ton andra ou ginosko" -- I do not know the man, which would indicate absence of premarital relation specifically with the betrothed bridegroom. Both indefinite article employed by King James and the absence of the article employed by Douay can connote either virginality at the present or virginality forever. But her future intention to remain virgin is seen not from this phrasing in itself but from her not assuming that the prophesied son will be hers and Joseph's.
You can only assume that she did not assume if you further assume that it did not occur to Mary to ask a very legitimate question: "Why appear to me now to tell me this?". You must assume that Mary did not think this pregnancy was imminent. I think she did and she was right. We know that she was pregnant within three months (three months being the longest time possible) of the annunciation because of her visit to Elizabeth.