Is it normal that I sometimes fantasize about women shitting?

Seems to me like you are a pretty normal guy. Most people thing of the strangest things, but rarely talk of it. So naturally everyone thinks it's as far from normal as you could get. If fantasizing about it is enough for you, there's no problem at all. The act itself is not normal, but sexual thoughts someone hides from others rarely is, so just stay with porn until you've found out more about your needs and if you would actually enjoy this in real life.