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I think the trick to defining sex addiction is to determine 'normal' sexual pursuits. This is an exercise in subjectivity as every person has their own comfort level and preference: some people want sex three times a day (or more), some have exotic tastes, and others favour vanilla sex.
It's too easy to say that people cheat because they're unhappy in their marriage. I've observed that there are a number of reasons why people cheat, and these have everything to do with the character of the person cheating.