This practice is employment discrimination based on sex. It is unequal treatment. Title VII focuses on the individual. It usually forbids generalizations about classes they may or may not be true about particular individuals, where the subject of the generalization is race, color, religion, sex or national origin. No legal problem.
Seniority is a legitimate defense.
In Wygant v. Jackson Board of Education , the Supreme Court affirmed that seniority takes precedent over affirmative action. This is especially true if employees were hired in a nondiscriminatory manner. Title VII forbids discrimination on the basis of sex. If a male applicant can prove the company has a policy of hiring only women receptionists, a prima facie case exists. The Equal Pay Act prohibits unequal pay for substantially similar work.