Thank you for your timely response and honesty. I do appreciate it and it has eased my mind tremendously. Thank you
Reacting first to just the title you chose for your question, no STD causes red-purple streaks of the thighs. Usually such "stretch marks" are due to other kinds of skin conditions.
Now having read the question, that's basically still my response. Clearly you have no STD, and I am not an expert on dermatology or the sort of symptoms you describe. (If a patient came to our STD clinic with such a story, we would refer him or her to a dermatologist.) In any case, no online expert can compete with the opinions from health care providers who have directly examined someone.
The direct answers to your questions are:
1) No, this doesn't sound like an STD.
2) As I implied above, your description sounds like "stretch marks", but that's only a guess. As a non-dermatologist, I also wonder whether your steroid cream is too potent or has been over-used. High dose steroids like betamethasone can cause thinning of the skin, especially if used repeatedly over a long time. I don't know that yeast or jock itch does such a thing.
3) Jock itch doesn't scar, to my knowledge.
4) See a dermatologist, if you have not done so; or if you have, return and ask him or her the same questions.
5) Not knowing what you have, I cannot suggest a remedy.
You could also consider asking your question of Dr. Rockoff on the dermatology forum. Tell him I raised the question of skin atrophy due to the steroid. In any case, put STDs out of your mind. Xlearly that's not the problem.
Good luck-- HHH, MD