Sci-Fi/Fantasy Book Idea Number 19

A young man from the country has awesome powers of magic. The only catch is that when he uses the powers his penis shrinks one whole inch (erect penis, that is). He teams up with a lusty wench who wields two giant swords and who has a giant rack of lamb in her lamb sack to save the world from an evil demon.

The two become lovers as people often do and she spends much of the latter half of the book castigating him when he uses his powers. Will they make it as a couple? Will he survive the journey intact?

Possible character names: Eunoch and Buxombra, Snippy and Moundine, Dinklage and Bombette.

Title suggestions welcome.