If a man's penis skin changes color and he doesn't know why, he should go to a doctor. He could have an infection, diabetes or cancer. A sunburn on the penis can happen quickly. If a man is outside without clothes, he will get a sun tan. That is normal. Too much sun causes a sunburn. Doctors say to use sunscreen (sun cream) before and after nude swimming. A sunburn can happen in a few minutes on a very sunny day.