Kantian ethics is fundamentally grounded in rationality and freedom, and to understand it one needs to think about category of the synthetic a priori that he develops.
Kant views ethical claims, which are grounded in the moral law, as cases of the synthetic a priori. This means that they tell us about the world, but are not grounded in experience. (Mathematical statements are probably the paradigmatic example of this).
Kant also, as stated in the question, does regard 'Man' as having a fundamental essence (though I don't think he puts it like this), that of a rational creature.
So when this rational creature thinks about what to do (reasons practically) as a rational creature it thinks as any other rational creature would. Any particularity, to its own interests or backgrounds, is a) a departure from pure rationality and b) a posteriori factors intruding - we do not have our background and interests a priori (note the similarity to Rawls's original position).
So a rational creature reasoning rationally will act as if the maxim of its action would become a universal rule, as per the moral law. Any other thought process would be a departure from its rationality (and thus freedom).
All rational creatures are thereby of equal 'dignity' (if you like) as loci and foci of ethical action. Treating them as means could only be a consequence of an a posteriori particularity on the part of the actor.
We can observe that Kant's conception, as grounded in rationality, is not limited to men and women, any rational being capable of itself reasoning is likewise included (e.g. if we were to regard as higher animals as rational in this way, or if we were to encounter intelligent aliens).