Hedonism
Noun
“The pursuit of physical pleasure; sensual self-indulgence”
Philosophy
“the ethical theory that pleasure (in the sense of the satisfaction of desires) is the highest good and proper aim of human life.”
Put simply, hedonism is when you value physical pleasure above other or all other things. It is a belief, like pacifism or communism, but not a faith like Christianity or Hinduism.
Hedonism, while considered “sinful” and “distasteful” in some religions and cultures, is not unlawful or considered any worse than the other beliefs listed above in modern society
Furthermore, your claim that both Polygamy and Polyamory are hedonism is bias and judgmental. In truth, polyamory is no more than having multiple people you love romantically, and while polygamy is outlawed in most legal systems, it has been a historically accepted act that has usually been used in religious settings, often involving a man having multiple wives. Usually, these multiple spouses are just for the sake of having children.