For God, only the pure and without blemish was to be offered. You think Mary could just have sex and bear other children in the same womb that carried Christ? How bizarre! We are worlds apart, indeed. God is not your next door neighbor who borrows your new tool and then you continue to use it. But I am beginning to believe that some Protestants see God as just the Good Ole Old Man next door that we can be "buddies" with.
Mary was pure and undefiled (in the sanctified sense that you are posing) ... when she offered herself to God for His purpose.
Afterwards, God left her with us ... as a continuing blessing to us ... to Joseph, to Jesus, etc.
God is not our buddy ... but He is our Father ... who gives every good and perfect gift unto His children.
Some people's calling becomes their life. Mary is one such person. She devoted her entire life to God. That much is clear.