She did not say “I will be a virgin”. She said, “I am a virgin”.
Besides, none of us were there. We only know what God, in his infinite wisdom, allowed to be in the Bible. I’ll go on what I read there. It is possible that she was a virgin her entire life. It is HIGHLY unlikely. And since no Bible tenets are impacted by it in the least, it is an utter irrelevance. It is like discussing what her favorite color dress was, with virtually NO evidence to go on.
Keep in mind, I am talking about AFTER Jesus birth.
The angel said "you will conceive" putting her mind in the future.