Both English translations are too delicate to use the word "sewer." This remarks is a bit too crude for them to translate accurately. One indication that this was said in humor is his use of the Greek word meaning "tossed out" that he often uses in his more extreme verses. However, the NIV's translated of the word for "sewer" to "body" is also unintentionally funny, equating our bodies with sewers.
Jesus's point is that the digestion of food makes it even more unclean, so eating what is unclean doesn't matter.
Christ's Words in Matthew as a Guide to 40 Days of Prayer.