My parents are divorced
Probably because monogamy is required by law.
Where is it required?
Monogamy ( ) is a form of relationship which required by law most places around the world. Plus who has time to be with more then one person. Hippies. 
Is polygamy illegal? 
Yes, polygamy or multiple mating is illegal and gross. 
Well it being gross is an opinion.
No, it's a fact. We are not animals that need multiple mating partners during a breeding season.
I hope you don't end up divorced.
Divorce
Divorce, also known as dissolution of marriage, is the process of terminating a marriage or marital union. Divorce usually entails the canceling or reorganizing of the legal duties and responsibilities of marriage, thus dissolving the bonds of matrimony between a married couple under the rule of law of the particular country or state. Divorce laws vary considerably around the world, but in most countries divorce requires the sanction of a court or other authority in a legal process, which may involve issues of distribution of property, child custody, alimony
often on the death of the first spouse or after divorce; they cannot have more than one spouse at one time because that would be polygamy which in countries with marital monogamy like the US is called bigamy. Polygamy is a form of marriage in which someone marries multiple people at a given time, and is illegal throughout the U.S. under the Edmunds Act. Part of the function of looking at marriage from a sociological perspective is to give insight into the reasons behind various marital arrangements.
Sociology of marriage Reasons for marriage.