There's a business necessity defense, and in sure a case could be made that a below median salary is a business necessity, considering addressing those marginal costs is exactly the intent of the business.
I think in practice that would be a poor defense especially when the burden of proof is on the company to show that they don’t have a discriminatory hiring policy. “Your honor, we ended up hiring mostly men simply because they are more cost effective” sounds like a really quick way to lose a disparate impact lawsuit.
Why would race makeup be affected? We're talking about the gender gap not the race gap. Unless women in tech are more diverse race-wise than men in tech.
And, BFOQ isn't relevant here because there is qualification that is relevant - it's just that if men are getting under paid/left behind, then merely offering a low salary would be enough to attract them.
Any business that doesn't make an effort to hire more women than is strictly economically efficient is begging to get sued. "We have very few women because men are cheaper" won't get them out of trouble. The mere fact that women are underrepresented in that company will be taken as proof of illegal discrimination.