You are right. If this gap exists the way liberals claim it does, then we should see all girl businesses all over the place. Such places would simply not hire men, due to their ability to pay females less.
I don’t remember if it was Sowell or Williams (probably the latter) that addresses this premise. I know Williams stated that the absence of such businesses is de facto proof that such a pay discrepancy doesn’t exist.
Sowell addressed it as “you’d have to believe that the business owners think sexism is worth the cost of the competitive disadvantage”.
And, yes, liberals WOULD assert that business owners, even black female business owners, are somehow so racist and sexist that they’d leave this competitive advantage on the table in favor of hiring white males.