When I first heard that Martin Luther believed in the perpetual virginity of Mary, it made me curious how and why many Protestants doubt this, so I started doing some digging. 

In my digging, I noted many doubters reference Matthew 1:25, "But he did not consummate their marriage until she gave birth to a son."

At first glance, this makes it seem like Joseph "knew" Mary (carnally) after she gave birth. St. Jerome tells us though that "until often denotes in Scripture...a fixed period, but often also an infinite time."

This can be seen in the following examples:

"And surely I am with you always, until the end of the age." (Matthew 28:20)
 
"For He must reign until He has put all his enemies under His feet." (1 Corinthians 15:25)

To be sure, in all three instances, the Greek word ἕως heós is used. I'm no Greek language scholar, that's for sure, but I do know that how we interpret the meaning of the word heós can make all the difference in the world.

For example, are we to suppose in Matthew 28:20 that He will leave His disciples at the end of the world?

Are we to suppose that in 1 Corinthians 15:25 that He will no longer reign after all His enemies are under his feet?

Obviously not, and this is why some would argue, as the Catena Aurea does, that "until need not imply a termination at a certain point of time, but may be giving us information up to a point from which onwards there is already no doubt."

Hopefully knowing all this was helpful in some way.

Last but not least, although what I'm about to say next seems to have relatively less support from the early Church Fathers, I found it very interesting so I figured I'd at least mention it.

Pseudo-Chrysostom tells us of Matthew 1:25, "It may be said, that know here signifies simply, to understand; that whereas before he had not understood how great her dignity, after the birth he then knew that she had been made more honourable and worthy than the whole world, who had carried in her womb Him whom the whole world could not contain."

Thus St. Thomas Aquinas remarks, "Otherwise; On account of the glorification of the most holy Mary, she could not be known by Joseph until the birth; for she who had the Lord of glory in her womb, how should she be known? If the face of Moses talking with God was made glorious, so that the children of Israel could not look thereon, how much more could not Mary be known, or even looked upon, who bare the Lord of glory in her womb? After the birth she was known of Joseph to the beholding of her face, but not to be approached carnally."