Most Helpful Guy
Well I'm not really going to argue what 'counts as cheating'. That's really up to the people who write the dictionary.
What I think this question is really about, is whether or when being interested in someone else is problematic, and that, I think, depends on what kind of interest exactly we're talking about.
When you're emotionally committed to a relationship, an interest in someone else will be little more than physical attraction or the appreciation of that someone's qualities in general.
It's when the interest involves an actual (serious) consideration (even if still only a fantasy), when things start becoming problematic. That doesn't mean it's wrong, but it means the level of emotional commitment is slipping. We have to keep in mind that emotional commitment is not something you can fully control, only really surrender to. If the feelings aren't strong enough, we can't emotionally commit, and if the feelings are starting to disappear, so will the commitment.
Now, in a (serious) relationship, it's important to make sure that you are emotionally committed. If for whatever reason you feel you no longer can be, then the relationship should end. After all, with the disappearance of emotional commitment, it already has in reality, and not being honest about that to your partner would be inconsiderate and disrespectful.
Now of course, all this isn't as easy to actually do as I'm making it seem, and it's not at all uncommon for people to make mistakes in this. What's really most important is that you always try to be honest, both to yourself and the person you're with, and to take responsibility for your actions and their consequences.