At a job interview the very first question they asked me after introductions was whether I have children. I said “yes, two” and the interview ended right away. The HR person immediately told me that I...

Asking job applicants or employees about the number/age of children, pregnancy, or child care arrangements is considered evidence of potential... discrimination, particularly if asked only of women. Such inquiries related to sex (including pregnancy) violate Title VII of the Civil Rights Act. To file a complaint, contact the EEOC within 180 days (or 45 days for federal employees).
U.S. Equal Employment Opportunity Commission (.gov)
U.S. Equal Employment Opportunity Commission (.gov)
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Key Aspects of the EEOC Complaint Process for Gender/Family Inquiries:
Discriminatory Inquiries: Questions about children or family planning are generally discouraged and can be used as evidence of illegal sex discrimination.
Differential Treatment: It is clearly illegal to ask these questions only of women and not men, or to use this information to make adverse employment decisions (hiring, promotion, firing).
Gender-Based Assumptions: Assuming a female employee is less committed or unable to travel due  more