I've seen these being misused on here more than once: presumptuous is not the same as presumptive.
Presumptive means based on a reasonable belief about what is likely to be the case. Hence the heir presumptive is the person likely to inherit a position or honour, but who may be supplanted by the birth of another person with a better claim. If Lord Thing has no children, his brother is the heir presumptive. If Lord Thing dies childless, the brother gets the title. If Lord Thing marries and has a son (we won't go into titles passing in the female line, it's too difficult), that son is the heir apparent - he definitely gets the title if he lives, and the brother doesn't.
Presumptive has nothing to do with good manners.
Presumptuous means behaving in a way you have no right to do and which may seem rude. If I ask you about your medical history, scrabble habits, and financial status, I'm presumptuous, not presumptive.