Given that the answer is "no" to the question you should be asking (which is "does it hurt?") the differential diagnosis is mercifully brief. A large, painless ulcer on the penis? There is one and only one answer. Our little treponemal friends are just setting up shop, preparing for what they hope is a long, fruitful (from their perspective) relationship with Mr. Happy's friends including the skin, the brain, the spinal cord and eventually the deep tissue.