Is it ok to have an innocent crush on someone else when in a relationship?

My colleague and I had a discussion as to whether it was ok to have an "innocent" crush on someone else when you are in a relationship.  My simple answer is "no" because you cannot control the situation and many bad things can stem from having a crush, even if it is seemingly innocent.  Here are my reasons:

Having a crush means that you are not completely faithful to your significant other.
You may not be physically cheating but you are mentally/emotionally cheating.  It means you are not 100% dedicated to your significant other.  Your diverted attention will likely take away from your partner and you may lose interest in your current relationship.  In addition, you may not be lying outright, but you may hide things from your significant other (such as not telling them when you hang out with your crush, not wanting your partner see your flirtatious texts).  Not being able to be completely open and honest will harm your relationship.  Please see my post on cheating.

Your feelings for your crush can grow.
What you thought was an innocent crush can potentially grow into strong feelings.  You cannot control if this happens, and it will make it harder for you to get over your crush.  The stronger your feelings, the more it takes away from your current relationship.

You want to impress your crush.
Even if you do not have the intention of pursuing something or you don't even flirt with your crush, you are consciously or unconsciously trying to impress them by seeming more intelligent, funny, charismatic, etc.  Or putting effort into looking more attractive, smelling better, being more visible to them, or contacting them more via text/email/calls.  This may lead to your crush suspecting you have feelings for them, or your crush eventually having feelings for you. 

A friend or colleague may find out about your crush.
Now you are threatened that they may accidentally or intentionally tell someone else in the group, your crush, or your significant other. 

Your crush may find out.
If your crush finds out you have feelings for them, if they don't reciprocate the feelings, imagine how awkward it may be for them, especially knowing you are already in a relationship.  Even if your crush does have mutual feelings for you, then the situation is worse because now you definitely would not want to get over your crush.  Now you have more reason to want to be with them.  This will only make it more difficult on your current relationship.

Your significant other may find out.
Imagine how devastated your partner would feel if they found out about your crush.  It would permanently change your relationship even if they eventually forgive you.  Once trust is lost, it is very difficult to fully regain.  Your significant other may always be suspicious you are cheating again, even if you are not.

Would you want your significant other to have a crush on someone else?
Think how you would feel if your partner had a crush on someone else.  You would not want your partner to experience that pain.

If your crush has feelings for you, then what would you do?
If your answer is you would prefer to stay in your current relationship, then having the crush can only be harmful to your relationship for the reasons above.  If your answer is you would prefer to be with the crush, then just end your current relationship because it is obviously wrong for you.