Monogamy assumes marriage is a natural thing people do. People are getting married later or not at all in increasing numbers.
I don't even think monogamous marriage is the main relationship style if you consider people that have affairs, divorced people, serial monogamist, etc., not part of monogamy. It's over represented in media but that norm has changed a bunch in recent years as well.
There are also tons of relationships that aren't marriage. FWB, poly, one night stands, etc.
I think the question could use a rework to clarify if you mean legally, socially, etc., as well.
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).
I'm married and poly. And yeah there are a lot of legal things surrounding marriage that I trust my nesting partner with. How to handle assets if I die suddenly, or what to do with me if I can no longer make decisions for myself, they're important things to have someone for.
I was going to ask one of my other partners to 'marry' me in the sense we'd have a celebration with partners, friends, and possibly family with the focus of the celebration being our partnership. They ended up very unexpectedly breaking up with me recently so that fell through but the thought was there :/