Compare today's office to the offices of 100 years ago. A woman in an office was rare, and paid less than her male counterparts. You don't think employers noticed this? By the middle of the century, the number of pink collar workers was so great that the standard pay had fallen. Or, rather, the purchasing power.
A similar occurrence happen in non-union manufacturing. In the 70s, women were taking jobs their husbands refused because they paid so little. Even factory jobs. When men finally had to take the jobs, the wages had denigrated.
It's not that women are paid less, today, but that men are, too often being offered women's wages to do a man's work.
Businesses can say they pay equal wages, but it's not women making men's wages, it's men making women's wages.
I hope I explained it right.