Mary said to the angel, "How can this be, since I am a virgin?" ([Luke 1:34])
Mary, whether she knew about Isaiah or not, in no way believe that she would conceive in the "normal" way.
This is a strange conclusion. The question you quote betrays the assumption on the part of Mary that for the Angel's prediction to be true, she would have a regular conception following a marital act. It is in the next verse that the Angel explains the supernatural character of her impregnation.
My thoughts exactly. Please see post #5947 above.