I presume it's a mis-presumed derivative for "thumb sucker" which the Yanks, such as the iconic B.A. Barachus utter as "'sucker".

A female of my acquiantence (he says carefully) is prone to the term "they suck cock big time", which always amuses me, as that seems my kind of people (gender and other basic attributes filtered to suite).

But it is odd, I mean ermm ... (may have to make up term) "pussy muncher" - could that be derogatory?

p.s. curse your rhetoric! :-)