It’s worth noting that polyamory and marriage aren’t mutually exclusive. Plenty of polyam people are married and even have non-legally binding marriage to multiple people. There’s a movement to make plural marriage legal because these people have the depth of relationship with more than one person that really should warrant the protections of marriage (like hospital visitation, legal protections re being forced to testify against a spouse in court, tax filing purposes, and child rearing, etc).