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.
 I won't we have been together 37 years we wont. We don't believe in the termination of a marriage or marital union.