Many people think you can truly love only one person and that once you fall in love, it should last for ever. Reality is different. You can get weary of anyone and what people consider real love is sometimes just infatuation that cannot last after two persons get two know each other better.
No matter how much you love someone, even if you have children with that person, you can find another one who seems to be equally right for you or even better.
Although romantic relationships with more persons are possible, people are usually furious if they find out that their lovers or spouses have an additional partner. They cannot even stand an innocent game of flirting which does not necessarily mean that the people are in love or want to live together.
It has come so far that if one spouse comes home later than usually or greets a person the other spouse does not know, the other one can suspect something and start spying. They even hire detectives. They themselves can play detectives as well by buying and installing surveillance cameras or programs that hack mobile phone calls. Then, they can find our when and whom the spouse calls, what they are talking about and where he or she is. Maybe this is sometimes crucial to find out that a relationship is going wrong and has to be terminated or improved. On the other hand, it can incite distrust as one spouse can catch another one arranging an appointment or having fun with someone else and jump to conclusion that this is infidelity. No one likes being treated like property of those who love him or her. Trust is supposed to be one of the most important ingredients of a healthy and fulfilling marriage.