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?
", the term "polygamy" is often used in a "de facto" sense, applied regardless of whether the state recognizes the relationship. In sociobiology and zoology, researchers use "polygamy" in a broad sense to mean any form of multiple mating.
Worldwide, different societies variously encourage, accept or outlaw polygamy. Of societies which allow or tolerate polygamy, in the vast majority of cases the form accepted is polygyny. According to the Ethnographic Atlas (1998), of 1,231 societies noted, 588 had frequent
(spousal support), child visitation / access, parenting time, child support, and division of debt. In most countries, monogamy is required by law, so divorce allows each former partner to marry another person.
Divorce is different from annulment, which declares the marriage null and void, with legal separation or "de jure" separation (a legal process by which a married couple may formalize a "de facto" separation while remaining legally married) or with "de facto separation" (a process where the