A clear line exists which must not be blurred. For each person there is an internal and external world. Action is the only bridge between the two. We own our thoughts and feelings and we answer to no one for them. Everyone would go to jail if held accountable for his or her thoughts and feelings. What you want to do is say that someone else’s thoughts and feelings have violated a social law, the law of relationship. Without action, that never really happened. Commitment is a pledge to do something regardless of how you feel. Based on your question, there is no cheating and there is no broken commitment. This is about assessing blame (to someone else) for their thoughts and feelings. If those exist as you suspect, they are a symptom of your relationship, which you and your partner co-created. You are both responsible, and both your actions will determine the outcome.

Thanks for your answers. What about spending time with people of the opposite sex, even if there are not "relations" happening? Dinner, drinks, etc and not including the one you are actually in a relationship with?

I think everyone has different interpretations of that, but as far as I'm concerned, if someone has different feelings, even though technically this may not be concidered cheating, it's not a good sign. If you're in a committed relationship with someone and then you're developing feelings, well, something is wrong. Maybe you haven't 'literally' cheated on your significant other, but you need to review your relationship and see what's going on. Cheating may mean just hanging out with someone you have feelings for. Yes, it's hard to see, but I still think you're betraying your significant other. But again, I think people have different interpretations of that word.