It's very easy to sit in judgement based upon one's own morals. Nothing is as black and white as people like to imagine. Wouldn't that be an easy world to navigate. In some cultures, monogamy is not the norm. I don't think anyone at the alter ever intends to cheat further on down the line but here are the problems IMHO:
People change over time. They may end up more as co-tenants, or good friends in a marriage, rather than sexual partners. They have a need for physical intimacy but not within their marriage. They end up looking elsewhere to satisfy this need, rightly or wrongly. People don't always want to throw away years of shared experiences and companionship and love for sexual freedom - one does not automatically eliminate the other. What if one partner decides they don't want sex with the other? Where does that leave the other person? Perhaps splitting up is not on the cards, or isn't wanted by either for the good of the family.
You can love, and desire, more than one person at once, in equal measure.
Human nature does not obey the civilised rules we would always like it to. We are programmed for two things - self-preservation and procreation. Affairs happen because human nature is stronger than the thin veneer of Western moralising that covers it.
Whilst marriage suits some people, it doesn't work for others. It takes a strong person to admit, from a young age that marriage is not their cup of tea and to resist the trappings of expectation from relatives/friends/society as a whole to get married, settle down, have children...……..and then live happily ever after.
Of course, most people don't want to be the villain who ends up cheating. But perhaps if modern society was a bit more accepting and transparent about human vulnerability and human conditioning, we would be able to navigate such tricky issues with more confidence and acceptance.