Soap may or may not really help the situation if you're getting irritated on "your bum". Have you thought about using a chamois cream of some kind? Do you have the right pair of cycling short?
But back to your question, I just use Dove shower gel. However, I try not to "overcleanse" my girly bits because that can upset the pH balance. I have treated saddle sores successfully in the past with a benzoyl peroxide cream and have used that, too, when I start to get some irritation on my bum or in the crease of my leg. I try to fend it off, however, with a liberal amount of Chamois Butt'r.
Live with intention. Walk to the edge. Listen hard. Practice wellness. Play with abandon. Laugh. Choose with no regret. Continue to learn. Appreciate your friends. Do what you love. Live as if this is all there is.
--Mary Anne Radmacher