Blaise Fragione, 38, admits that in October 2008 he knocked out the victim, named only as "F.", with a blow to the head, severed most of his penis with a razorblade and threw it down the toilet.
Fragione says that he "lost it" after F. came to tell him he was having a relationship with "Mado", his partner of 14 years and mother of his two children.
"The sudden revelation of this adultery made him lose control," said Fragione's lawyer, Marc Ceccaldi. "He is well aware that he caused a significant trauma."
Fragione faces up to 15 years in prison if found guilty of "aggravated assault accompanied by mutilation".
The victim maintains that Fragione forced him to go to his home, where he tied him up and mutilated him.
F. is "extremely fragile... his life has been completely destroyed," said his lawyer, Jorge Mendes Constante.
The shortening of his penis to three centimetres (just over an inch) when flaccid and five centimetres when erect had resulted in "a deterioration of his self-perception, affecting his social, interpersonal and sex life."
The victim is awaiting surgical reconstruction.
His former lover has returned to her original partner, whom she now plans to marry.