You can often tell and know if it's a friction burn or an STD by the appearance and the symptoms.
When you have a friction burn it often appears as redness and mild swelling which is often localized in a certain area of skin irritation that is caused by rubbing.
When you have an STD it will often present with other symptoms like multiple sores, itching, unusual discharge, tingling feeling around your genitals or even burning sensations.
STDs that lead to rashes often require medical attention to treat and diagnose properly and if you suspect you have an STD, especially if other symptoms are present you should see a doctor.
Most friction burns mainly cause pain or discomfort when touched in the area affected and an STD often includes the other symptoms including burning, pain during urination, unusual discharge and itching.
Friction burns are caused by the physical rubbing or friction of your skin and STDs are transmitted through sexual contact with an infected person.
If you experience any genital sores, unusual discharge, burning or pain or unexplained itching, especially if it's accompanied by fever or swollen lymph nodes you should see a doctor.
Or if any irritation persists longer than a couple of days or gets worse despite care at home or if you have recently had any unprotected sex then you should see a doctor.