BlueRose is spot-on. There is an innocent child involved in this mess who had no choice into when (s)he would be born, and to whom (s)he would be born TO. This is not a concept. This is a fact: children do not have any options, and you may have produced another innocent life, as well.
I agree that you're both not at a level of selflessness to commit to a healthy relationship - for yourselves, and for your children. Dissolving the legal contract of marriage seems to be the most prudent option, and I would recommend that you both avoid entering into another relationship or contract of marriage for the foreseeable future. You both sound as if you need to grow up. I would also suggest that you use a condom or opt for a vasectomy before you produce even more offspring for which you will be legally responsible for at a minimum of 18 years.
DNA testing would also seem prudent, at this point. With the divorce will come custody/visitation/support matters, and determining your level of responsibilities is a legal imperative.
If both of these children are determined to be yours by DNA testing, it will be of benefit to those innocent lives to remain civil, rational, and supportive of the children. I would recommend all parties engaging in parenting classes and sorting out how to put the ADULT issues aside for the sakes of the children.
This situation is not about what you, your soon-to-be-ex-wife, or your girlfriend wants. Its about doing what is right and proper for the sakes of innocent lives that were produced without any say in the matter.