PolyOz

The core concept of monogamy as used today is of exactly two people in a sexually and romantically exclusive relationship. This relationship is substantially based on exchanged promises of sexual exclusivity - whether or not these promises are kept. A common form today is "serial monogamy", wherein there may be multiple monogamous relationships over time, but the participants are supposed to have no more than one partner at a time.

The roots and earlier meaning of monogamy was "one marriage", referring to a legally and socially recognized marriage. Today it applies to serious relationships with or without legal marriage. Our society now has some space for discrete multiple relationships, but not for multiple marriages (which is often called bigamy).

Note that Polyamory is not the only alternative to monogamy, or the "opposite".