Most, but not all, people enter a relationship that is based on assumed monogamy.
Two-timing hurts. Infidelity can destroy families, crush spirits and cause pain. It gives rise to questions as to why it happened which generally leaves feelings of inadequacy, low self esteem, lack of satisfaction with existing relationship, why the need to seek thrills outside the relationship. It begs the question: if the relationship is so bad, why are you in it? But often people on the receiving end don�t know that it is that bad. Hence the shock and hurt.
Often, people who cheat don�t want to admit relationship failings themselves, thus we have the old �in vino veritas� as Uncle Caesar would say.