It's a very rare person who doesn't fantasize about other people when they're in a committed relationship. It's normal to have a healthy curiosity about someone other that who you're partnered with.
However, it's not cool if it doesn't remain a fantasy. If you start obsessing over someone else, dropping hints and comments, or surfing porn when you should be paying attention to your partner, that's a recipe for disaster.
Unless you're both mutually comfortable with disclosure, keep your daydreams to yourself. Most fantasies should be exactly that and nothing more. Often, playing out fantasies will actually ruin them, because it's never quite the same in real life.