We don’t actually see any nakedness or sex in The Crucible, but we do learn that Abigail Williams and the rest of the girls liked to dance naked in the woods while they contacted departed spirits.
John Proctor lusted after Abigail Williams while his wife was sick and they had passionate sex in the barn (which Proctor indicates was the appropriate place for that kind of activity). Though he does not seem too ashamed of his actions, Proctor accuses Abigail of being a whore, and his wife claims she is a harlot. Later, however, Elizabeth Proctor admits that perhaps her own cold nature drove her husband to adultery. Regardless, the only real sex occurs off-stage, in a barn eight months before the story starts.