God did not demand it. It is not God's perfect will, but His permissive will. God's perfect will was for the natural to be changed into the spiritual through obedience. Sin is what made it necessary for the natural to be sacrificed. Some of us are trying to offer up spiritual sacrifices to God before we have sacrificed the natural.
Sanctification means more than being freed from sin.
It means the deliberate commitment of myself to the God of my salvation, and being willing to pay whatever it may cost. If we do not sacrifice the natural to the spiritual, the natural life will resist and defy the life of the Son of God in us and will produce continual turmoil. This is always the result of an undisciplined spiritual nature. We go wrong because we stubbornly refuse to discipline ourselves physically, morally, or mentally. If you don't, you will ruin your entire personal life for God.