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.