It is so funny that you bring up this topic.

When Tiberius was here (a one year old Bengal kitty) he would clean Juni's butt all the time. In fact, when I re-homed him to Beth, out of state, that was the first thing I said about him. That he was into licking butts. I have no idea if he is performing such friendly deeds with/on his new buddy, but Juni now does it to Chloe. No one would dare touch Miley's butt. Anyway, I think it is an animal thing and short of keeping them apart, or yelling at the cats, it will continue. They also clean each other's ears and that can't be fun either. Think about it. They have rough tongues!

Juni and Chloe now fight too much to spend their resting time with each other. All they do is chase, attack, role and restart, so there is not much time for cleaning each other.

I feel your pain, when spying the event in progress. I usually say something like "and then you lick me with that mouth...." then move on to some other impossible question/answer about cats.

SAS