I'm going to poop in the designated no pooping bathroom in my office at work
Why would you use a restroom inappropriately?
Because one of my co-workers does the same thing.
It's understandable to want fair rules for everyone, but its rude to break a rule yourself just because others are doing so. You might end up getting reprimanded for your actions.
Fuck, this isn't fair, why do only I have to follow all the rules.
You follow rules for yourself not for others. It's not your concern what others do. If your coworker is breaking rules you should ask him to stop it or file a formal complaint for his inappropriate use of the bathroom.