What causes white or skin-colored bumps, noticeable only on erection and when the penis is stretched out?

Hello doctor,

I have had these lines on my penis for the last two years. Before puberty, there was only one line, or it was not noticeable. I do have unprotected sex, but only with partners I personally know. My ex-girlfriend had consulted a doctor, and she was fine. I have done the research, trying to figure out what it is. I have had to look through so many pictures, and I do not see anything like it. It looks like stretch marks on the outer side of my penis head, but what started to worry me was when my skin started getting drying out. Maybe just growing up, I always had dry skin, but there were almost tiny white or skin-colored bumps, unnoticeable unless it is erect and stretched out, but I do not have symptoms of any STD or STIs. Besides this bump no discharge, and occasionally I get a random pain, but I have had four sexual partners in the last three years with which I was in relationships, and all were tested negative.