I think you should tell your wife that you've gone out with the OM's wife several times, and that, although you realize that you don't love the OM's wife she is going to have your baby. Let your wife know that you'd like to raise the child in your family.
Make sure your wife knows that you think the OM's wife is not suitable as a mother, but you would like to continue seeing her for the occasional tryst.
On the other hand, maybe that's terrible DB advice. Scratch that.
