PARIS, March 14 (AFP) - One of France's biggest-ever criminal trials, in which 66 men and women are accused of raping and pimping out children, has shed light on an under-reported phenomenon -- the existence of female sex offenders.

The image, still very much widely accepted in society, of the woman as wife and loving mother -- who often assumes the role of physically and emotionally protecting her child -- has helped keep women sex offenders in the shadows.

"The portrait of a sexual aggressor is one of a man with a penis," explains psychologist Philippe Genuit, who works at the women's prison in Rennes, the largest of its kind in France.

"There are very few studies on female sexual aggressors, as it's a phenomenon that no one dared imagine just a few years ago," Genuit adds.

"In general terms, the only mother is a good mother. Maternal closeness with a child is somehow less worrying than paternal closeness," says Paul Bensussan, a psychiatric expert who works at the appeals court in Versailles.

Nonetheless, 14 women have been accused of sexual crimes in the trial that opened in Thursday in the western French city of Angers. The victims were aged six months to 12 years at the time of the alleged rapes and sexual assaults.

Bensussan says that in addition to societal tolerance of a mother's close relationship to her child, it is generally accepted that a sex assault by a woman is not as physically traumatizing as an assault committed by a man.

"The male aggressor penetrates, the female aggressor fondles," he notes.

Experts say that women who commit sex crimes are also less likely than men to issue threats of other physical violence.

"Such incidents (involving women) often take place in an atmosphere of fusion, of tenderness, but that does not rule out the psychological destruction of the child," says long-time prison psychiatrist Sophie Baron-Laforet.

In the courthouse, women are often accused of complicity, with men generally considered as the driving force behind the actual assault.

"Sometimes the woman is present but does not participate. She can sometimes manipulate the child for the man. In rare cases, she can assault the child but as compared with men, it's a tiny number," notes Roland Coutanceau, a psychiatric expert at the Paris appeals court.

But Genuit says the image of the "passive, submissive mother is changing" as a result of trials like the one unfolding in Angers, or the so-called Outreau case, which saw 10 defendants convicted of sex offenses and seven acquitted.

Incestuous mothers are often vulnerable women with meager financial means and who are starved for affection, experts say. In the vast majority of cases, the women were victims of sexual abuse, either as children or adults.

That does not mean that the upper classes are immune to cases of incest, but Genuit says the situation in such families is "more opaque", as "family loyalty and the concept of legacy are very strong".

"These families have a lot to lose if a victim comes forward," Genuit adds.

The Angers trial also raises the issue of parents pimping out their own children, another phenomenon long ignored or kept under wraps.