Most famous French erotic novel apart from de Sade

Lesbians engage in cunnilingus, in a scene by Édouard-Henri Avril. Source: Wikipedia
The theme in "Gamiani" - the title is from the surname of the heroine- is largely lesbian. A young man inadvertently witnesses a lesbian scene, whereupon he comes out of the cupboard and joins in. Most of the book is occupied with the ensuing love triangle. One scene has Gamiani climbing onto the erection of a just hanged man. The plot ends with the two lesbians taking poison and experiencing orgasm and death simultaneously. A happy tale.