Welcome to the STD forum.
We don't normally look at posted photos on this forum. But I broke our own rule and took a quick look.
The good news is that no STD causes a rash like you describe or that I see in the photos. The bad news is that I don't know what it is and won't speculate; this forum is strictly limited to STD issues, not for resolution of non-STD genital symptoms. Further, if a dermatologist can't figure it out, there is no way you should expect any distant online expert, on this forum or any other, to come up with a definitive answer. All I can say is that most of the many conditions that can cause rash anywhere on the body sometimes do so on the genitals; don't assume an STD just because of the location.
That said, I'm rather surprised a dermatologist didn't have an explanation for you. You might return to that office; or seek out another dermatologist's opinion. But in the meantime, I wouldn't worry much about it. This doesn't look serious and for sure not an STD.
Regards-- HHH, MD