My wife is pregnant with another man's child

I am so sorry you had to go through this pain. 😔 It is definitely obvious that you are an individual that is not into polyamorous relationships, so i too follow what majority advised you which is divorce. In this case a lawyer comes in handy. Though, it is still up to you whether you want a divorce or give her another chance. We also do not know the backstory to your problem but it is best to be respected with privacy.