Workplace promotion is withheld from a woman based solely on gender
This argument applies a generalised assumption about all men and all women ('men are more reliable') to make a specific employment decision about an individual woman. It is a hasty generalisation because it draws conclusions about an individual from a group stereotype without any evidence about that individual's actual performance or reliability. It also assumes client perceptions based on gender, which is itself a form of systemic gender bias that employers have a duty to address.
The manager might say: 'It's not about the law; it's about what's best for our clients.' How should you respond?
After you give your response, they may push back. Here is how to handle each counter-argument.