No, checking people out ISN'T cheating
Here's my response:
As long as you don't stare repeatedly or verbalize your attraction toward the person in a flirtatious or overtly sexual manner, you're fine.
Let's face it. We might love our partner to death, but it's natural at some point to feel attracted to someone you come across at work, the mall, the grocery store, or the gas station.
And there's nothing wrong with that. It's just nature. We can't help but feel entranced by a good-looking man or woman.
The trouble comes when a person makes a deliberate attempt to see and interact with the individual more often. For example, you know that Sally in accounting tends to be in the cafeteria around noon, so you intentionally "walk by" the cafeteria every day on your way to the restroom.
I'm not saying you necessarily have to avoid the person or not be friendly when you encounter them. But investing time and effort in investigating where he or she might be at certain times and strategizing ways to increase communication can easily become a slippery slope that leads to cheating.
In such a scenario, a person's hormones cloud their better judgment. Just because we find someone hot doesn't mean we should seek their validation (e.g. get compliments out of them) to feel better about ourselves.
If seeing other attractive people has this great an effect on you, it merits a hard look into your relationship or marriage.
Do you still feel attracted to your partner? Are your physical and emotional needs not being met? Do you find yourself on the cusp of an affair?
If you wouldn't want your partner engaging in such behavior, then you shouldn't do it either, out of respect for your relationship.
Merely checking out a person isn't cheating so long as your thoughts aren't consumed by the individual. We've all seen shows and movies where a man is having sex with his wife but pretending she's someone else. Once it gets to this point, cheating suddenly seems a lot more palpable.
Remember, kissing or sleeping with someone is far from the only form of cheating. You can also cheat on someone mentally and emotionally, both as potent if not more so.
If you find yourself struggling to look the other way and resist the urge to get closer to the attractive man or woman, maybe you're just not feeling it for your partner anymore and should end the marriage or relationship before you're caught in the act.