John keenly observes on the facebook-
Very few people ever say what you’ve said here, Michael, but which seems to me evident: that no one would have translated parthenos as virgin unless there had ALREADY been a virgin-birth tradition. It can’t have been read out of LXX Isaiah but must have been read into it. So the question why early Christians thought Jesus’ mother was a virgin must have some other answer than Isa 7, and it’s time the issue was addressed.
He is referencing TM Law’s post here. Barton’s comment is indeed interesting and his challenge is, I think, appropriate. Perhaps it is indeed time to take a substantive and in-depth look at the tradition rather than simply repeat old arguments.