Hi,
This red rash may be a case of fungal infection. Fungal or yeast infections may favor areas like the groin and other warm and moist areas in the body. Yeast infections are not necessarily sexually transmitted. Direct skin to skin contact may aid its transmission. I suggest that you avoid unnecessarily touching the area as this may only further irritate the skin. Another differential is a dermatitis. Would you be able to point out known triggers for the condition? Is the area itchy?
Avoid any intimate contact until you have this assessed by your physician.
You might just have sensitive skin down there. It is actually quite common. Buy yourself a penis health creme containing Vitamin A and E and you should feel some relief within a few days. These are made for guys and help with the very issue you described above. Good luck.