The rash could be fungus or just irritation. You should try over-the-counter clotrimazole 1% cream twice a day for 2 weeks. That would get rid of any fungus (which is not very contagious if at all, by the way.) If the rash persists, show a doctor, though this hardly sounds like any STD.
As for "stretch marks," even if that's what they are, stretching is not the cause in many cases: athletic teens get them, as do people who use powerful cortisone creams in the area. A doctor can clarify the diagnosis, though not much can be done if that's what they are.
Best.
Dr. Rockoff
I do you use a powerful cortisone cream. It must be the cause of the stretch marks. The cream does not do a very good job on the rash. It just controls it. You said it is not an STD, but it still is just bugging me. All it is is a rash that appears to caused by rubbing. I have seen pictures of herpes and those are blisters that are painful. And my rash is not painful. Thanks for our help.