I'm fantasizing about a coworker
What is causing you to fantasize about them?
She is really hot and she is also single. I want to fantasize about her.
It's okay to have a fantasy about someone. You shouldn't act on it though since most workplaces have a policy against dating in the company.
Wtf that is no fun
There is not problem with having fantasies about someone. There are many relationships that start out in the workplace and work out. Just be careful though, because if it does not work out it could really make your working situation more difficult.