first of all, impeccable personal hygiene, and a sense of hygiene is critical and i consider it alomost an intrinsic part of sexuality and sex. If your guy "doesn't wipe himself very well", and smells of faeces when he's nude you shouldn't just reprimand him but rather not even let him near you! Let him know it's important to you if you are so concerned about bacteria. which brings me to my second point: a clean, freshly washed healthy anus might as you point out have waste in at any given time, however its not the part of the body with the highest concentration of bacteria - the bigtoenail area is. Large numbers of bacteria are present on and around us all the time. The mouth, too.
Suggest bathing with your lover before going to bed, and show him how important and what fun washing properly can be. Relax and enjoy yourselves.
But will someone please answer my original questions on masturbacion anal ?