Full Idea
Non-descriptivists (e.g. prescriptivists) reject descriptivism in its naturalist or intuitionist form, because they are both destined to collapse into relativism.
Gist of Idea
If morality is just a natural or intuitive description, that leads to relativism
Source
Richard M. Hare (Universal Prescriptivism [1991], p.453)
Book Reference
'A Companion to Ethics', ed/tr. Singer,Peter [Blackwell 1993], p.453
A Reaction
I'm not clear from this why prescriptism would not also turn out to be relativist, if it includes evaluations along with facts.