eating ass is my favorite sexual activity. it's one of the primary reasons why i infrequently have sex. (it goes w/o saying, you shouldn't just bury your face in unfamiliar booty).
it's this thing i have about symmetry/contrast that turns me on: compared to women, men are angular in shape. so i appreciate features on a dude that break up the hard angles and, dare i say, soften him a bit: his eyes, smile and "the curvature". nothing like a masc, fit guy with a big butt and a smile. #poison. i also just like the bounce of it against my face.
anyway, it's been years since i had sex w/ someone i didn't know for several months, so this informs my answer to your question for sure, which is that i don't normally use a barrier. just feels odd.