To be honest, this is not a major issue for me. I think the plain reading of some passages would seem to go against the idea of Mary having only one child. I don't think any of my doctrine rests on Mary having another child. However, the idea expressed by RobbyS, that Mary could not have other children or the Incarnation is void is a bit curious to me and I have a large problem with that.
I don't think that's an orthodox statement. It would be a different reality than the one we have, but I don't think it would change who Jesus is.
What would be weird for advancing the idea of a Virgin Birth would be to point to the lady with 8 kids and say, "The first one, little Jesus, Virgin Birth, He was."
It is certainly easier to accept a Virgin Birth as an only child and especially if Mary was a consecrated virgin, as some think.
SD
No, my idea is that the contention by Helvidius that Mary had other children undercut or at least diluted the notion that Jesus was divine. It fits the Arian notion that Jesus was a kind of kuku in the nest.