My wife is pregnant with another man's child

Talk to your wife, She has to understand your concerns. Even if that ends in a Divorce. Ask yourself, do you want the kid to be a constant reminder of her infidelity? In future, do you imagine yourself in a family where if your own kid asks about the other child, you would be confidant enough to answer? I think an end to this relationship might not be the only solution if you can get a handle on yourself.