What do cyclists use for contact dermatitis?

You’re going to laugh, but these work well. For light cases, use either calamine lotion or A&D ointment. Yes, you read that correctly. These reduce the irritation and let your body heal itself.
If these don’t work, a mild steroid cream such as hyrdrocortisone.
If the rash is caused by something like poison ivy or poison oak, then use a product that will remove the irritating oil. The best stuff I’ve found is Technu. Works like a champ.
But the best treatment is prevention. Immediately take off cycling clothes after a ride. This will allow the sensitive areas to dry. If you are getting saddle sores, I recommend a lubricant. Vasaline and Bag Balm are effective, but very messy and difficult to clean. There’s a product called chamois butter. Some people use some kind of fat. It makes sense since fat is the natural lubricant in your body.