OK, let's be rational here.
You can safely dismiss all the 'fippant' responses, this is clearly upsetting you, and not a matter for jokey answers.
I'm sad to say I am inclined to agree with the previous posters who suggest that, at the very least, all is not well with your marriage, and at worst, your husband is seeing someone else.
I would not dream of contacting, much less seeing anyone else involved, this is between the two of you.
Pluck up your courage, and tell him you want to talk to him. Look him straight in the eyes and ask him if there is anything he wants to tell you. This may be just the trigger he needs to confess- the strain of an affair can be enourmous, and he may be hoping in a perverse way that he is found out so he can start doing something about it.
When you have talked it over, and been angry and upset, you then need to think for yourself about how you want to proceed - either with some counselling, or if you feel your relationship cannot be salvaged, you need to examine your future options.
It may be that your husband has been unable to discus his feelings about a new baby, but an affair, or even a 'friendship' is absolutely not the answer.
I hope you can work things out - but the sooner you start, the better it will be for you. There is nothing to be gained by hanging on when the evidence is certainly there that something is seriously wrong.
Do please keep us posted - there are plenty of people on here who will support you through this.