Am I responsible for child support if wife got pregnant by another man while we are still married?

The child is presumptively yours until you file a denial of paternity. If you know the name of the actual father, name him and ask that the court order a DNA test. You will both be tested and whoever is the bio father will then be declared the father. This can't be done until the child is born.

In Louisiana, you are presumed to be the father of children borne by your wife. Therefore, unless the biological father of the child signed the birth certificate, you will need to file to disavow the child within a year of learning of the child's birth. Additionally, if you don't plan on reconciling with your wife, I highly recommend getting divorced to avoid future problems like this from arising.

It used to be true but is changing. You need an attorney, and a divorce.

When you file for divorce (and I would not wait one minute), claim that you are not the father and that you want a DNA test to prove it. Generally, if it shows that you are not the father, then you will have no obligation to pay child support. But until you are divorced or you prove somebody else is the father, you are the legal father.