My wife is pregnant with another man's child

You should talk to your wife and tell her clearly that it is not going to work this way. It really is going to turn a very messed up situation and as your wife will progress into pregnancy, she too will go through emotional upheavals, and you never know how she would start behaving with the real father of the baby. You must sort out things as soon as possible by telling your wife you do not want interference from the other man.