Hello. It's good that u raised a question about your sexual thoughts. first of all let me tell you, cz u r just 20 years old the sexual thoughts are normal.masturbation is normal.it becomes OCD when it crosses a certain limit.
Sexual thoughts from a very early childhood is not usual but it's not completely abnormal too..many times sexual thoughts appear unknowingly.its a part of childhood development.scientifically speaking,there thoughts arise in every human being(male n female) in varied intensities.
Next Steps
if u think that the thoughts are so disturbing that u r not able to control it at any cost,n controlling them causes more anxiety or trouble in you then this can go towards obsession spectrum..
daily masturbation is normal,if it's more than twice thrice a day,we must go for some extensive therapy n treatment
Health Tips
the least here u can do is,go to a public place immediately when u get the thought,avoid watching any sexual content on phone.
stay among people for as much time as u can.
start cultivating it hobbies which involves Ur complete attention.like playing a sport with a partner or getting into painting.
if the thoughts are still disturbing n troubling, u might need an extensive therapy and treatment.u will have to consult a psychiatrist.