I agree with the collective. I think it’s perfectly normal to have “crush” feelings, even if you’re in a relationship. There are different kinds of crushes, too, some more platonic than others. I also have crushes on completely unattainable people, such as Tina Fey. :P
I’m always totally honest with my girlfriends and I will tell them if I have a crush on someone. I’m very monogamous and when I’m in love with someone, they’re the only person I want to be with, and I also make sure this is known. Harmless crushes, ones that you have no plan or desire to actualize, are perfectly normal and fine, in my opinion. They can even be kind of fun, and I enjoy sharing them with my girlfriends so they can tease me or whatever. Same goes with their crushes.
It’s when your heart truly begins to stray, you fantasize about being with that person, or start trying to spend extra time with them, that you begin to tread a fine line.