Letting them call it a 'Preferred' pronoun allows them to seem 'polite'.
Forcing them to use the term 'Mandatory' shines a light on the power play.
Force them to use the correct term.
"Oh, are you suggesting that you have the right to issue a MANDATORY pronoun?"
And don't accept the word gender. It's sex. Letting gender into the conversation allows for the flexible and arbitrary shifts that a deterministic word like sex does not.