Whoa! Red flag! It would be different if he didn't lie about it. This is so unfair to you and your baby. He should be focusing on you and his new child. I would sit down and talk to him about how hurtful that is and how it's not good for the baby. Ask him why he has to hide things. Sometimes, innocent conversation can turn into something deeper and become emotional cheating. Some men (and women) don't realize that is what they are doing. Maybe it's like a midlife crisis kind of thing? I would bring all of that up and if the behavior continues (and this is what I would do) I would probably seperate, but that may be extreme. I guess it depends on how incriminating everything is.
oh also there are those men who basically become jealous or feel left out during pregnancy, maybe he was feeling that way and started getting attention in other places? Anyways, sorry you are dealing with this, hopefully it was just a time of poor judgement. :p
and no, I don't think you are overreacting....