Use a soap dish.

Maybe the marble needs to be sealed. Marble needs sealing and is a porous stone. It might not work, so soap dish recommended is a good idea. I have a marble counter material as a niche shelf in my shower and it does not stain but I don't have soap sitting on it and I wipe it dry after every shower.

I don't use bar soap anymore. I have a bottle of Mrs. Meyer's sitting on my honed cararra (sealed) marble vanity counter. Even when I miss a drip or two, no harm.

It's the nature of the marble beast. If you so much as breathe on that stuff, you'd better wipe it up right away.

No, marble lovers do not want Corian. If they liked it, they would buy it. They lust for stone. That dissing stone schtick is very old.

Could be oil in the soap. For oil/fat removal from marble, you can try a 48-hour poultice.

Let the marble breath for a few days without anything on it and see what happens. From now on, just use a soap dish or switch to liquid soap/bodywash.

Good marble is awesome and you don't need to tip toe around it. You just get to keep things simple - don't use harsh chemicals. A diluted bleach in a spray bottle is all you really need to clean it.

Remember that there are marble bathrooms in Europe that are older than the United States. It's not the delicate flower that some people will have you believe. It just might not take well to the modern chemicals that so many people use in their bathrooms. :)

A Turkish Hamam (bath) built in 1583: