The announcement refers to a pregnancy right then, not in the future. Note she became pregnant and was known to be pregnant before they were married!
Really? What is that future tense doing there?
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?Now explain this. The Angel speaks of the future. Mary responds with the present time, as if her not knowing man now precludes her pregnancy in the future. That can only be if her status as virgin is not going to change after marriage. This is exactly what the tradition tells us: that she was a temple virgin, given to Joseph in a custodial marriage that was to preserve her virginity.