Relationships are about trust, agreement, promise, intimacy, security, connectivity and liberating growth together. We as individuals have needs and dreams of our lives, and when you are unfulfilled in yourself, or your partner, finding physical/emotional/intellectual intimacy is one way in which we seek out what is missing, that, and orgasms are terribly addictive. In many instances, infidelity brings relationships closer together, whether the infidelity is discovered or not, as it often concentrates the individual, or couples thoughts and actions towards "making it work" . People don't have these sexual excursions because they are bad people, but because they are trying to move towards a fulfilled life.
I think viewing "cheaters" in such emotionally charged, reactive, stereotyped and generalized terms is very short sighted, and not allowing for a deeper understanding of the complexity's of the human mind, experience and relation to others. Labeling people homewreckers and cheaters is just the scared lizard brain reaching in fear to find its club in the dark, so as to smash the threat, all without thought, but satisfied in itself with simplistic, reactive non-solutions.
At the core of us all, there are needs to be fulfilled, however one cares to define, or place value on them, or themselves.