Based on the rules and TOS guidelines, sexual/nudity/gore is allowed if it's:
A) relevant to the plot of your stories
B) are not there for shock-values.
Even if you do decide to show them, you have to have the NSFW tag on for these subjects.
Like, you can have a sex scene in your story if it's relevant to the plot, but you can't have a sex scene in there with the purpose of creating a pornography and to arouse your readers. Same goes for gore.
As for nudity, that actually is ok to have with or without a NSFW tag (someone can correct me if I'm wrong) because Tapas recognizes that nakedness is not inherently sexual. But if it's is shown in a sexual like, it has to be behind the tag, no questions asked.