"I eat a lot of ass, it affects my nutrition." (Filthy Frank)
It's a mite paradoxical with me. In that I love eating ass as much as I love eating pussy but the girl's hygiene must be above any reproach. Because as much as I adore the female rectum, I have a violent aversion to fecal matter. If I ever got married, it would be stated in the prenup that I never have to change the spawn's diapers.
I dunno, man. It just occurred to me naturally. Interest in performing an actual rimjob only became attractive to me later but I was always turned on by the prospect of going to town on a girl's buttcheeks. Like when I'd see a girl with a hot ass in a thong as a teen, I'd think "Mmm, I'd kiss and lick that all over".
