Females are paid less, on average, not due to any kind of discrimination, but because they genuinely work less [produce less output per unit time for the employer] and because they generally like to work in less demanding and less skill-hungry jobs. Maternity and also monthly cycles combined with limited physical strength/stamina/endurance and reduced other abilities mean lower productivity. Female employees are also costlier to maintain - extra security has to be provided due to their inherent vulnerability to female-specific crimes, and so on.
Naturally, in order to maintain the "cost per unit of work received" parameter, they must be paid less per month. A fact that everyone - including women themselves - knows but people fear stating publicly is that women are hired primarily to add glamour/oomph for the real workhorse male co-workers and/or for the predominantly male audience/customers [whether it be in the military or in a strategy consulting firm or a hotel or an airline or in a movie], or they are hired when the employer needs a worker who works 0.5-0.75 times a male worker and is paid 0.5-0.75 times. Not hiring anyone means 0 work and 0 pay - not acceptable. Hiring a male means 1.0 work and 1.0 pay. But what if the production need is only 0.5-0.75 units of work? Here's where women can sometimes fill the gap.
So if you force companies to pay women equally, they'll see little, if any, benefit in hiring women. Why get less bang when you're gonna spend the full buck, after all? Women employment will fall [unless, of course, like every other place, women beg for quotas to compensate for a lack of an economic case for hiring them].