I would have to agree with Corlan, Patience. I don't think anyone would have any reason to 'side' with your exH pregnant girl over you, apart from on the basis of the information that you have provided. It is your account of events that is informing people's comments, and so you should take their comments in the spirit in which they were intended.
You feel that perhaps you should not have posted, but maybe you should take some time to reflect on what people are saying...I did not notice any 'vitriol' in the posts, but some women get offended when their 'sisters' are made out to be scheming and manipulative by men who just want to shag them and then sod off, without giving proper thought to the consequences. The fact that you love your ex is not mutually exclusive with recognising his role in this.
Of course there is the possibility that this girl was using no form of contraception, and planned to get pregnant, but your exH himself has admitted that he did nothing at all to assure protection, so he is still equally responsible.
You have said that you want to support your ex, but surely as a friend you should be helping him come to a proper understanding of his part in what has happened, otherwise, what kind of relationship will they have as a family (albeit a family living apart), when he holds her entirely responsible for his mistakes? As long as their relationship has this blame at its foundation, it will be hard to manage.