If my wife is pregnant by someone else, am I legally responsible to take care of this child?

Under the laws of all states in this country, under the facts that you have written about you are presumed to be the father of the child that you have written about and are obligated to support the minor. Your recourse is to file a paternity action against the presumed true father and if proven, he would be responsible for the upkeep of his child.

I suggest that you consult with a family law attorney further to give you guidance as to how you wish to proceed in your matter.