It's not the sort of erotic novel with graphic descriptions (though being concerned with conquests and desire, it is still labelled érotique, but de Laclos is often called cerebral). Language/tone vary according to the various correspondents, but it's altogether not very explicit, that I recall.

Libertin means "emancipated", strictly speaking, freed from the established order, especially religious morality (the association between sin and sexuality, or virginity to purity etc.). It's only later that the word came to take the narrower meaning of sexual transgressions, while originally it was meant more as a philosophical standpoint, similar to how we use anti conformist now. Some Libertin novels are much more anticlerical than erotic. Some of them are proto-feminist. It's was a realist/humanist current, concerned with depicting things as they were rather than how the Church wanted them to be. A lot of it is erotica, but how explicit it was varied massively.

This novel is late in the game. De Laclos attacked both excessive prudery and pure libertinage, you might say.