Whatever floats your boat - don't worry about it kid!
It is harmless enough to fantasize, no matter what the fantasy is. Acting out, you have to be sure your partner is willing, but fantasy harms nobody. Chubby gals everywhere will thank you for thinking they are sexy! But as gyndandee says, acting this out could give you a lot of bacteria, so you might have to leave it in the realm of fantasy.
It's normal to fantasize just don't be touching the feces or licking the anus as you can get very,very sick.