Perpetual virginity?

/ posted in: Religion

I read a post today at The Color Purple that discussed a point I’ve always been confused about. Why do people, especially in the Catholic Church, discuss the perpetual virginity of Mary when it clearly states in the bible that Jesus had siblings?
I even pulled the bible off the shelf today to look it up to make sure I wasn’t the crazy one. It wasn’t hard to find the verses. They were listed in the concordance.

Mark 3:31 “Then Jesus’ mother and brothers arrived. Standing outside, they sent someone in to call him.”

Seems fairly easy to understand to me.