Much is made of Mary's virginity, so of course sex SEEMS to the point. If I'm wrong, then tell me so.
You are wrong.
The stain, originated in Adam and Eve, our first parents, is disobedience to God. In Christian belief, all descendants of Adam and Eve have a nature weakened by hereditary sin.
There is no suggestion that Mary was conceived by anything other than natural sexual relations between her parents, or that there is anything sinful in normal sexual relations between spouses.
The contention is the Mary was free from the hereditary effects of sin, in order to pass a sinless human nature to Jesus. Jesus was conceived miraculously, not through sexual intercourse, in order to be the Son of God rather than of a human father.
Disobedience starting with Adam and Eve.
Mary did NOT disobey God, but instead said “Yes” without which we would have no Christ.
Have you ever read the account of the angel Gabriel’ s annoucement to Mary (Annuciation) in Luke?