It is allowable only in circumstances where it is a "bona fide occupational qualification reasonably necessary to the normal operation of that particular business or enterprise".
A gentlemans club can get away with it. Someone who simply wants to change their gender ratio can't.
Discriminating against accountants is legal in all circumstances.
Title VII of the Civil Rights Act of 1964 prohibits discriminating based on sex except when it is a bona fide occupational qualification. Diversity is certainly not a bona fide occupational qualification, so they can't recruit based on it.
They may be justified in taking diversity into account in the hiring process, but they cannot exclude candidates based solely on sex. In Regents of the University of California v. Bakke, SCOTUS held that candidates can not be excluded exclusively because of race. In Grutter v. Bollinger, they decided that it was legal to take it into account, but only on an individual basis, not to exclude candidates altogether. These both deal with race, but I see no evidence that sex would be treated differently since the same law applies to both.
Civil Rights Act does not ban affirmative action. As long as its aiming to correct a statistical imbalance, which a 10-1 ratio is certainly is. (And a few more points, but nothing to conclude your assertion is certain)
As the two cases I cited indicate, affirmative action is not illegal, but completely excluding candidates because of their membership in a protected class, or any type of quota for minorities is illegal. Affirmative action can be applied during the hiring process to favor some candidates, but not to completely exclude some class of candidate.