When faced with a situation where a minor pretends to be an adult and has sex with an adult, adults should take the following legal self-protection measures: 1. **Confirm identity**: Before having sex, try to confirm the other party's true identity and age. You can ask the other party to show their identity documents to ensure that the other party is a legal adult. 2. **Clear communication**: In the process of interacting with the other party, clearly discuss and understand each other's age and willingness, and record relevant communication content (text chat records, etc.) for subsequent needs. 3. **Legal consultation**: If the other party pretends to be an adult and causes legal consequences, consult a professional lawyer in time to understand your legal rights and obligations. 4. **Call the police or seek help**: If you suspect that the other party is a minor, call the police in time or contact relevant agencies (such as youth protection agencies) for help to prevent the situation from further deteriorating. 5. **Keep evidence**: Keep all relevant evidence materials, including chat records, call records and possible photos, in case of emergency. 6. **Psychological support**: If you suffer psychological harm, you can seek help from a psychologist for psychological counseling and support. In any case, when safeguarding personal legitimate rights and interests, one should abide by the law and avoid taking extreme actions.