Is it ok to have a crush while in a committed relationship?

I want general opinions on this. To me, a crush is harmless as long as your intentions aren’t to get closer to this person than you are with your SO. Also, it isn’t necessarily bad IMO if the crush is kept secret. Now, telling this person that you’re into them would be a complete red flag. It’s just like saying “Keep Going”. What do you think? Are crushes acceptable during a relationship? Is it basically already cheating? Or is it just a red flag and something to nip in the bud?