Every now and then, when I'm going about my duties as a Catholic seminarian, someone will accost me and demand to know why Catholics say the Virgin Mary never had sex. I like to respond by calmly explaining that is what the word "virgin" means.
There are a few times in the New Testament where they speak of Jesus' "brothers", that being the generic Greek word for "relatives". Ancient Greek is less specific about relatives than English; Latin, on the other hand, is more specific about relatives than English. But by and large, the people who are confused about the topic of Mary's virginity are not Scripture scholars, so they don't know about these passages.
No, the reason why people seem to have trouble with accepting Mary's virginity seems to be a cultural thing, as our society likes to promote sex as the highest value. To promote virginity, as the church has always done, is counter-cultural to these twisted modern-day values, but we must remember that as Catholics, we are called to preach the Gospel in and out of season.
The Blessed Virgin was a virgin, before, during and after the birth of Jesus Christ, and her virginal pregnancy is clearly represented in the Bible as the miraculous work of God the Holy Spirit. Some may be inclined to doubt this, as St. Joseph once did, but to deny the virginity of Mary is contrary to what has been divinely revealed, taught and believed by Christians in all places at all times. Therefore, let us not doubt or alter the Scriptures to suit our own tastes, but instead let us pray that God the Spirit will continue to work within us, granting us the wisdom, understanding and humility necessary to accept this teaching.
Mary, Seat of Wisdom, pray for us.