They didn't "miss" something; it was a non-issue. They simply believed that Mary remained a virgin. However, you will not find Protestants running to pray to Mary because there is no original Church teaching on this. I would suggest that all this devotion to Mary is a new teaching after the Reformation that has spun totally out of control. Now we see images of Mary on grilled cheese sandwiches, moldy tunnels and corn flakes even though no one knows what she looks like. Sorry if this sounds like anti-Catholic hate speech.
I believe that the images of Mary on grilled cheese sandwiches and whatever are nonsense, the whole issue reminds me of an old Johnny Carson episode when he interviewed a woman who saw people like Abraham Lincoln in potato chips.
Marian apparitions, however, have been noted throughout Church history and they often seem to be to people who have no preconceived notion or bias.
And why is it so important now for Protestants to believe that she didn't? Scripture is silent on the subject of her having other children. Surely Luther, et al were aware of the verses about the Lord's "brothers." Other than a desire to minimize Mary, is there one good reason that some Protestants insist that she had other children?