As you say, most Protestants don't believe Mary was ever-virgin.
Well, I must be in the minority because as a Lutheran I was taught all my life that she was a virgin.
Lutherans have vacillated a good bit on the "ever-virgin" issue, and there is actually a lot of back-and-forth on the issue. A pretty good summary is here:
http://paulmccain.worldmagblog.com/paulmccain/archives/019796.html
I'm Lutheran...and I think its possible she had more children.
I also think its a matter of little signifigance.