Does permanent use of sildenafil destroy my dick when I'm older?

My urologist prescribes me sildenafil but everytime he makes me feel bad and asks me how i think i will do it in 20 years (i'm in my 20s). What is he suggesting? I can't find anything about sildenafil loosing it's potency after long use. He even compared me to pornstars but as far as i know they usually get ED from using stronger and partially illegal drugs. Please someone enlighten me. Am i making my ED worse by using Sildenafil?