While I will grant you that each gender was doing the work they were most suited to, it was not equal work. Not even close.
The only argument in favor of it which made sense was that a young lady with a good rack in a skimpy orange top was more equipped to slow or stop traffic than her male counterpart. She, therefore, had a talent in short supply and was entitled to the same pay for easier work.
Unions do that also. I did some work for a big hotel in DC once where they had some unions. Telephone operators got the same pay as pot scrubbers regardless of the ease of the job. Of course, most phone operators were female and men did the pot scrubbing.