The marriage bed is undefiled. Scripture says so. Do not call “unclean” what God has said is clean.
If Mary remained a virgin or not...SO WHAT?
She became the Mother of God, in which work so many and such great good things are bestowed on her as pass man’s understanding. For on this there follows all honor, all blessedness, and her unique place in the whole of mankind, among which she has no equal, namely, that she had a child by the Father in heaven, and such a Child . . . Hence men have crowded all her glory into a single word, calling her the Mother of God . . . None can say of her nor announce to her greater things, even though he had as many tongues as the earth possesses flowers and blades of grass: the sky, stars; and the sea, grains of sand. It needs to be pondered in the heart what it means to be the Mother of God.
(Commentary on the Magnificat, 1521; in Luther’s Works, Pelikan et al, vol. 21, 326)