It’s a country based thing:

• US: “I’ll know it when I see it”
• Japan: “anything goes as long as the genitals have censor bars”
• EU: range from “not without written consent and photo ID of the drawn character”, to “anything goes as long as it doesn’t look realistic”
• China, India, Arabs, Africa: “no porn, go rape someone”

The combination of instance owner, server location, and client location, determines whether something is legal or not.