I personally believe that it is okay provided that you do not intend to do anything about it. Having happy crushes is okay because it may brighten up your day. It can make you feel better at work too or in school. Just remember that if you start flirting with your crush, this is the time when having a crush may be wrong because you are acting on your attraction towards someone.
You may not be unfaithful yet but if your crush happens to like you back, what are you going to do about it? It is best that you know your boundaries and you remember to respect your relationship with your partner. You may also assess yourself why you are getting even more attracted to someone when you are already committed.