I want to draw a distinction between two sorts of belief (I'm sure this isn't original to me, and I've used this distinction many times before).
Some forms of belief are simply knowledge, they are extra pieces of mental furniture inside our heads.
Other forms of belief are directly action-guiding; that is, they involve a response of the whole body and provoke a whole repertoire of behaviours.
I call the second form of belief 'weight bearing'; they are forms of language that actually 'do work' in people's lives. (When I spoke about understanding the grammar of religious faith it's this second aspect that I have in mind - so often atheists deal with religious belief as if it was entirely the first sort of belief).
Examples of the first sort of belief are (for me) details of quantum physics. I can read in the newspaper or in books about scientific experiments that are establishing different forms of sub-atomic particles, like the Higgs Boson and so on, but this is simply extra information. It doesn't have any impact whatsoever upon my life. If tomorrow the scientists turned round and said - actually we've got this wrong, the Higgs boson is actually two different things (and both at the same time ;-) - then I'd find it interesting but no more.
An example of the second sort of belief, however, would be: my 1 year old daughter would be hurt if I dropped her on her head. That is action-guiding in a very strong sense, and I actively try to be as careful as possible when holding her. There is no disconnect between mental idea and human behaviour.
Or, a different way to bring out the distinction, consider the difference between 'Mrs Smith is committing adultery' and 'your wife is committing adultery' (and female readers can reverse the gender). One has an impact upon a life, the other does not.
Now what sort of belief is the Virgin Birth? If it is the first sort of belief then I don't really have any sort of problem with it - it's simply an historical curiosity. It's like pondering the likelihood that Jesus had black or very dark brown hair, dark brown eyes, and was unlikely to have been more than about five and a half feet tall. Those thoughts are more or less true - but nothing hinges on them.
The trouble is that weight has been placed upon this doctrine - it has been made to do work - and that is where my concerns are really focused.