It is reasonable to ask what difference does it make--Jesus is the Christ in either case; both ways are spiritually comforting--one, Mary has a miraculous birth; the other, Jesus is born like any other "son of man." In either case, Mary learns to be a holy Mother to Jesus, much as Saint John was deemed to be His mother: "Who are my Mother and my brothers?" (Mark 3:33).
The Immaculate Heart of Mary is still as Saintly and pure, regardless of whether she and Joseph had sexual intercourse and other children, or not. Her purity is perhaps more remarkable in that latter scenario, and even serves to give more hope to us all, much as does the conversion of Saint Paul. In either case, it is neither Jesus nor Mary nor Joseph nor John nor Paul whom Christians believe on: "He who believes on Me, does not believe on Me, but on Him Who sent me." (John 12:44; that is imho the more literal or faithful translation; some translations interpolate "...does not believe on Me *alone*").
If it comforts you to believe Jesus was the son of a virgin, then that is helpful; however, if Jesus were born of sexual congress, would that make Him any less a Savior? With God, all things are possible (Matthew 19:26); so what if Joseph were the biological father of Jesus? It shouldn't affect any Christian's genuine faith and love. Must we conclude that God couldn't have commissioned the Savior if He were born of Joseph and Mary? He was at least able to set apart Paul, while Paul was yet in his mother's womb...(Galatians 1:15). The only way the Savior could come down from heaven is by virgin birth? Is God that limited? Did not the Holy Ghost descend upon Jesus, during His baptism, at the age 30? The Holy Ghost certainly came down from heaven onto others at Pentecost, who were conceived by their biological fathers and mothers. Again, what difference does such theological opining make, if Jesus is Messiah?