Lord of the Flies is about a bunch of pre-adolescent boys, so, nope: no sex here, although there is extensive nudity and a few poop jokes.
We would, however, like to draw your attention to a highly disturbing scene in Chapter Eight when the boys kill a female pig. The sexual violence of the scene and the repetition of "right up her ass" (8.197) make us think that this slaughter sounds a lot like rape. Since the only females on the island are female pigs, we don't have to face the horrors of what might have happened had little girls been stuck on the island, too. But Golding does make us wonder.