And why not actually?
Look, here I have two subordinates, a boy and a girl. Basic duties are well dealt with, but there is one additional thing: from time to time you have to drag some weights to the other end of a large building. Not a very heavy thing, but the girl will not pull.
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The question is for you, man. The guy is carrying something heavy. What does the girl do at this moment? It is free, i.e. Has her working day ended? Or does she still do something else, not physically heavy – just another job (reports make, for example)?

In the first case, you are right. A guy works when a girl is resting and needs to get more. In the second case, sorry, you are a sexist. The girl also works, and not the hery falls. If the work is mental, then her duties may be even more difficult at this point than his. But for those hours she gets less.
In this case, there would be two options: once you have found a guy a job that the girl is not busy, find a girl a job that the guy will not be busy. It’s not necessarily physically difficult – you can just give her what the guy doesn’t like, and let him never do it. Everyone will appreciate it, and it’s fair. And the second. Hire a truck, man! A man, even if he is at least three times a man, is not obliged to bury his health on some mythical "additional employment." He did not hire a bag here.
Or you can buy a truck, and let them drag together. Riding is easier than pulling.