I would give you advice but the picture link does not workany longer.
If a Mark showed up 4 months ago and is still there, it can not be an STD.
STDs needs to be ruled out, of course.
Things to consider would be syphillis, a genital wart, herpes, benign skin lesions, or molluscum contagiosum.
Appropriate testing with blood tests, swabbing the lesion, or possible biopsy (if the diagnosis continues to be non-revealing) should be discussed with your personal physician.
I would consider a urologist or dermatologist referral.
Followup with your personal physician is essential.
This answer is not intended as and does not substitute for medical advice - the information presented is for patient education only. Please see your personal physician for further evaluation of your individual case.
Kevin, M.D.
www.kevinmd.com
FYI: It has been that same size since it showed up.