If the ratio of those hired matches the ratio of applicants how does anyone make a complaint of discrimination?
No it's not. It's in fact the opposite. Sexism is systemic to the point where you're lucky to have 5% of your applicants be female.
There are real differences there between sexes, and those preferences affect what career is the best for you. You don't pick whats average for you, you pick your best career with all things considered, and small differences in preferences can skew the outcomes heavily. Also I like to point out there is bunch of careers where there is heavy shortage of MEN choosing it for same reasons.
They'd complain that the company wasn't doing enough to attract resumes from underrepresented groups. That's the usual thing, anyway.