Why must Christ Jesus be born of a virgin? Is…

Why must Christ Jesus be born of a virgin?
Is sex in marriage a sin?
Why did the bible say in sin did my mother conceive?
Does sex in marriage make conceived baby a sinner?Psalm 51:5.
Why could he not be born by a married couple?
What exactly makes a child that has never committed sin a sinner?
If there is why a man is conceived in sin or born a sinner, why couldn’t the same reason make Lord Jesus be conceived in sin or born a sinner ?