When a person chooses to build a life with another person, they generally get married. It is often a horrible shock when a person finds out their partner is also married to someone else. This is a major betrayal of their marriage vows. It suddenly negates the entire relationship for the partner that finds their marriage is not exclusive. The one person they trusted above all others has betrayed them completely.
There are many reasons bigamy occurs. Sometimes people in a marriage simply walk away without bothering to severe their ties legally. This is nothing more than laziness on their part and often leads to more complications in their future. Other times, people that are bigamists choose to have two partners. They may be seeking financial support from one partner and using it to support another. No matter why this occurred, it is truly a betrayal of the marriage and all it means.