Is penis dermatitis an std.

Not usually. needs to be seen but most STD's are actual lesions not dry skin, ulcers, or discharge from the opening. I would see urologist or dermatologist for answers.
Need exam. Unfortunately, outwith a history and physical, I cannot tell what this dermatitis is. You will have to be examined and possibly tested by your doctor to make sure it is just a dermatitis and not a symptom of another disease.