Hate to break it to you, the sponge was for their butt. And, in Asia they didn't have a sponge they just used a stick.
> The researcher Gilbert Wiplinger put forward a theory on the use of the xylospongium and it seems much more credible. He suggests it was used for secondary cleaning of ancient lavatories in a similar form in which modern toilet brooms are used.
> The discovery of scraps of cloth in an ancient septic tank in Herculaneum led also environmental archaeologist Mark Robinson to conclude that scraps were used for wiping instead of a sponge.
Take a look at the Spa Sanitas Per Aquam conference proceedings from 2009 for more details.