I am not permitted to enter into a "more money, less leave" contract with a willing employer.

Women (fortunately or unfortunately) are not permitted these contracts either. It's not possible to promise chastity or childlessness or plead hysterectomy and get more money; the actual condition of one's reproductive equipment or desires is ignored and instead a broad bias ("she might leave!") is applied to all.

Right, but that's unrelated to the efforts to get men to take up more leave (increasing the "use it or lose it" for men), since even if the time was all usable by the mother, you couldn't enter into a contract renouncing it either. So I don't see how it's germane to the point you introduced earlier.

Yes, but fundamentally you don't understand the premises your interlocutors are starting from. Essentially, the view you're encountering is that a certain amount of vacation, parental leave, and related benefits are an intrinsic good and that nobody should be forced to choose between their career and, for instance, taking paternity leave. To someone with that view talking about losing the "choice" to forego these benefits for the sake of your career is pointless in much the same way it'd be pointless for me to appeal to the Bible in an ethical debate with an atheist.