This rash has spread somewhat. I now see an area of redness on the head of my penis, and on the upper part of my right thigh. I also see some small red patches above my belly button. I do not see any blisters, but more of a red rash, somewhat raised in certain areas.
Given that it's on your neck and above your belly button now, I'd say this isn't herpes, and probably not shingles.
Herpes infects nerve groups, and genital herpes infects the sacral ganglia. That covers the boxer short area, and probably wouldn't go above your waist. It definitely wouldn't go on your neck.
Oral herpes, if you had gotten hsv1 both orally and genitally (if either woman had performed oral sex on you, which you don't mention), infects the trigeminal nerve, which doesn't include the neck in the areas it affects.
Shingles is typically on one side only.
I agree that it's probably an allergy or fungal infection or dermatitis of some sort. It's good that they cultured it, so you'll know for sure it isn't herpes, but I really don't think it is.
Have you used a new soap, laundry detergent, shampoo, dryer sheets, or anything that comes into contact with your skin? Have you eaten any new foods? Any foods that may have changed their ingredients (think "new and improved flavor" and the like)?
Allergies can develop at any time, at any age. Even if you've used something for years, you can suddenly become allergic to it. Keep working with your doctor to see what you can find out. Did any doctor suggest taking benedryl to see if that helped?
This rash seems to be spreading further. Now I itch and see some red irritation on the left side of where my thigh meets my pubic area, as well as a small patch or red skin about seven inches below the lowest part of my armpit. Does HSV act this way, spreading so quickly and in several different areas?
No, herpes doesn't spread like this. You might have more than one spot of blisters, but it wouldn't act like this.
It also wouldn't be underneath your armpit or on your torso. I explained above about how it infects nerve groups, and you'd only get it in the boxer shorts area or on the face (nose and mouth).
A visual diagnosis of herpes is wrong a lot. I don't know an actual statistic on this, but I'd guess at least 30% of the time.
Nothing you describe resembles herpes. It could be poison ivy, an allergic reaction, etc. Can you take benedryl? If you can safely take that, and have time to sleep for a few hours, that may help with the itching.