Luther also espoused the perpetual virginity of the Mother of God, as did, surprisingly enough, Calvin and Zwingli. The attempt to completely remove the Mother of God from the record (Mary reduced to a mere "vessel", or the notion that she and Joseph had further children) appeared centuries later in protestant thought and belief.
Protestants as late as John Wesley in the 17th century accepted the Perpetual Virginity of the Theotokos without any question whatsoever. They also tended to refer to her, I believe, as the Mother of God (I know Calvin did) as well. That was standard until quite recently (last couple centuries).
When I found this out...I was shocked, and felt betrayed by my Protestant upbringing! It actually played a large role in my digging deeper into Holy Tradition, and ultimately my conversion to Orthodoxy.
Okay, so which Protestants believe in the perpetual virginity and which don't?
Anglicanism officially believes in the perpetual virginity. Given the recent theological atomization of Anglicanism, it's hard to tell what "Anglicanism" means anymore.
Confessional Lutherans believe in the perpetual virginity.
Do most Reformed Christians still believe in the perpetual virginity? How about Arminianist traditions?