Under NC Law, if a man's legal though separated wife has an affair and a resulting child by another man, can the legal husband be held responsible for child support for the child?

If the divorce hasn't taken place. Which it shouldn't before the child is born then the husband is the presumed father until the courts determine otherwise.

So if you don't initiate court proceedings to determine paternity ASAP you may found yourself responsible for support.

Yes since they are still legally married, the father or the bio father needs to clear this up as soon as child is born in court