Welcome to HCM! PREMATURE EJACULATION is a common medical problem and one tends to contract the pelvic muscles (also responsible for contraction of anal sphincter) during masturbation; the “pressure-like” pain which you are experiencing at the beginning of masturbation could be because of this. Ejaculation is an involuntary process consisting of rhythmic contraction of pelvic muscles along with contraction of PROSTATE & SEMINAL VESICLES to expel the semen out of body; the fact that you do not experience any pain during this process indicates that your prostate is healthy. I would suggest you KEGEL’S EXCERCISE by voluntary contraction of muscles to hold back urine or defecation, once identified you can do it standing or even sitting when trained; it will enhance the size and intensity of erections.