Hi
With a significant sexual history, a urologist will be the doctor to see first. He will help rule out any infections and yes, help identify if the rash or skin condition may be related to sexual activity. I would suggest an initial assessment by a urologist so that underlying infections if present may be addressed.The urologist may refer you to a dermatologist if the rash may be more of a localized skin condition.
i would also like to add rashes do crust over if i dont keep them moist and tend to appear only on the right side on the penis only close to the tip, not all the way along the left side so to speak.