This is a little bit more complex.
Sexual desires are not a constant for each invidual person.
It seems (in the poly community) that awesome sexual experiences with one partner make one want more sexual things with the other partners rather than less.
Not telling is mostly about wanting to avoid the other party getting angry.
I wouldn’t mind disclosing the reasons to someone if I was given some confidence they wouldn’t get angry at me.
Thus most of the time one ends up using polite safe generic to turn away people.