If Mary was not a virgin, there would be a lot of doubt about the Immaculate Conception. For Mary to remain a virgin means there were no siblings. This means the focus remains on Jesus. Imagine how the dynamics of the Bible would possibly be altered if Mary had loose morals or other children. There would be no miracle of the Immaculate Conception, which means Jesus would not have been viewed as the Son of God. The churches would not have much of a congregation.
