It is normal and natural to find someone else attractive even when you are in a relationship. Fanatasizing them is no harm, however, it is necessary to draw a line between attraction and obsession.
Romantic relationships are like a glass house, which can break easily if proper care is not taken. You and your partner do all you can to build a strong relationship, untouched by any misunderstandings, disloyalty and trust issues, miscommunications and every potential outer force which can shatter your relationship. However, there are bitter and unavoidable truths that couples tend to avoid because it might disturb the harmony in their relationship.
You might find yourself surrounded by inhibitions that you don't want to talk about with your partner, one of them being, thinking about someone else, fantasizing about someone apart from your partner. The mere thought of someone else coming into your mind, apart from your partner, fills you with guilt and shame. You start doubting your own loyalty and you fear for being less committed to the relationship you have. Those constant battles which you go through, of having someone else in your head leaves you with millions of unanswered questions.
However, do you feel that you are not loyal to your partner just because you have someone else in your mind? How many times have you imagined kissing your favorite film star, after watching some of their most romantic movies? Thinking about someone else, and fantasizing them while you are in love with your partner does not mean that you are cheating on them. It only means that you have admiration for others too, but you have love only for one person, and that is your partner.
It'd be called cheating when your fascination about someone else starts to influence and affect your present relationship with your partner. This kind of notions and thoughts mostly come if you have been with your partner for the longest of time. Ocassionly, you find yourself thinking about the guy you saw at the coffee shop, and how disturbingly good looking he was. Your partner is the most beautiful and attractive person on this earth for you, however, that does not make others any less attractive and beautiful. There are more than 7 billion people on this planet, do you really think it is unfair to think that there is no one else attractive than your partner in this world? That's practically impossible, right?
You should understand the fact that fantasizing and acting on it are two different things. A person can run away from the world, but can never run away from themselves. You are adult enough to understand what infidelity means, and what dreaming and thinking about other people means. With time, not only does your relationship mature, but you do too. And unlike your teenage days, you have learned to control most of your impulsive tendencies.
What makes you more scared about sharing intimate moments in your mind with a stranger, is the fact that you know you will not be having those moments in reality. However, the irrational part of your mind fears that you are being lenient with all those thoughts that might be considered as cheating because you might end up indulging in it, but it's not true.
Humans aren't wired to stay attracted to just one person. It is normal and a natural thing when you find yourself gazing at someone else, who fits under your definition of attractive and hot. These things do not result in lessening or influencing the love you have for your partner and certainly does not mean that your relationship is on the verge of crumbling. It only means that you are just being human. What might ruin things between you and your partner, is when you start tagging these fascinations about someone else as cheating or disloyalty or micro-cheating.
The matter of truth is, you are being normal, and the thoughts and fascinations toward other attractive people are normal too, whether or not you are in a relationship with someone. This is just how our mind works. If at all this leads to misunderstanding and creates trust issues in your relationship, then you should be able to educate your partner about how basic this is and how they might also be indulging in such situations, and talk things out.