Normally with Anal Sex there is no feces to worry about but if an "accident" does happens that is what soap, water, and the shower is for afterwards.
And yes, as mentioned above there is a fetish for "poop sex" which is COMPLETELY DIFFERENT than being into Anal foreplay and sex. Those into the "poop fetish" will use the feces as a lubricant but hard-core fetish lifestyle stuff like that is for a small percentage of people usually not connected to the rest of us into Anal Sex.
Done right, Anal Play & Sex will always top Vaginal-always. Once the woman is taught what it can do for her and the man realizes that her Anus is as tight or tighter than a hand-grip then it can be understood why couples into ir are SO INTO IT.