The Children of Mary?
Where are Jesus' brothers and sisters at a time like this?
When your Protestant friends bring up the "fact" that Mary had other children because the bible tells me so, ask them to check out the early Church fathers. Ooops, too Catholic! Well then, just ask them to check out the writings of their founders, Calvin, Luther, Wesley etc. They all held to the perpetual virginity of Mary and were very Protestant.
Go here to read an excellent cogent defense of the Blessed Mother's perpetual virginity.