Treating people as things is where most evil starts, and employees are real people not things. As real people employees have human natures, and human nature isn't Kantian universalism or Nietzschean will-to-power or whatever: human nature is social, human beings are raised by mothers and fathers in families, and not everyone is a father at all let alone is everyone equally a father all at the same time. To hire a father is to hire a person who has primary responsibility for materially providing for his family; such a hiring is a different kind of thing from hiring a teenager to mow the lawn or hiring an older mother with an empty nest looking for some extra cash to spend on the grandkids.
Employment as an institution which treats a father of five as a fungible productivity unit equivalent to a bachelor, or a single woman, or even a wife and mother, is a deliberate institutionalization of inhumanity. Deliberate institutionalization of inhumanity is a moral evil, so the institutionalization of equal pay for equal work is immoral.
That doesn't imply that in every case a woman should make less money than a man, or any such risible extrapolation. It doesn't mean that a family-man slacker should draw more pay than a diligent spinster. Human beings being what they are, exceptional circumstances are common and varied, judgement of individual circumstances is always required, and few things are more inhuman than "zero tolerance" categorical rules about the nuts and bolts of everyday life as actually lived.
But as some kind of categorical employment imperative backed by the force of law, the concept of equal pay for equal work is fundamentally inhuman and immoral. There is a basic difference between treating people as human beings with inherent dignity and treating them as interchangeable fungible productivity units, despite how amusing it is to say "fungible productivity unit".
I understand the objections: it is presently illegal to hire and set pay based on marital status and children, it is difficult to get employers to do the right thing, if fathers are morally entitled to greater pay - a living wage - than those who do not have the garnering of a living wage as their natural duty, well, capitalism as presently consitituted is going to lock fathers out of the workplace, fragment jobs into contract work and piecemeal jobs, and hire the cheapest workers. I get all that.
So much the worse for how things are presently constituted.