Is it normal to like the smell of ass, and to enjoy women pooing?

No honey, it's not normal. You must've been a pooch in your former life. It's true that people are drawn to certain weird smells but I think it's something going on in the brain that you just can't help. Just like people who ingest metal even though they know it's dangerous. It's called Pica.
You can either give in to your fantasy or seek medical help. I suggest you at least try the latter.

Sometimes a fantasy is best left in the imagination.

If women didn't want men to think and fantasize about their butts then why would they wear tight yoga pants, leggings, tight jeans, short skirts, short dresses, short shorts, and lingerie and underwear that shows off the buttocks area? All stylish articles of clothing that accentuate and draw attention to the buttocks, thighs, hips, legs. Women's clothing is so sexualized and women gladly do continue the objectification of their bodies by the fashion industry and by themselves, yet they have the audacity to accuse men of being pigs? Women enjoy being stared at by men even though they try to hide the rush of excitement it brings them. Another thing...women always wear makeup, lip gloss, perfume, all stuff that promotes and maintains the illusion of feminine "perfection". So it's all the more rewarding and satisfying when women are caught off guard, when they're coming fresh out of the shower without having had time to alter their appearance, or when they had a long day at work and are too tired to bother or care. But ironically that's sometimes when they look their best because they're all relaxed and casual. And it's totally understandable for a guy to fantasize about women sitting on the toilet. Because of the forbidden fruit factor. Guys wouldn't think about women's butts so much if women didn't constantly act like their butts don't stink!