What is cheating? I think ultimately if you've made a commitment to a person there are plenty of types of cheating. Physically or emotionally, however most people don't react so bad with emotional cheating.
In my opinion - cheating is when you're committed to someone and you physically do something with another person, whether it's just a kiss or it's sex. It's cheating. I can't speak for everyone though.
What do you feel? Betrayl, anger, sadness - plenty of things. It's not exactly the most wonderful feeling in the world, however raw it may be.

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To love is to suffer. To avoid suffering, one must not love; but then one suffers from not loving. Therefore, to love is to suffer, not to love is to suffer, to suffer is to suffer. To be happy is to love; to be happy then is to suffer, but suffering makes one unhappy; therefore to be unhappy one must love or love to suffer or suffer from too much happiness. I hope you're getting this down.