Imagine you are not dependent on your current partner in any way. You are financially stable (let's say you a multi-millionaire). Either you don't have kids or if you do, somehow they're not dependent on you (or something).
Now think of your favorite media personalities that you have a crush on, and imagine one of them meets you in person and takes an elevated level of interest in you.
If you don't have any celebrity crushes and that there is no one in the world you find more attractive than your current partner, I give up.
If there is, would you be tempted to cheat? If not, I give up.
If yes, I rest my case.