The Zeta do not marry in the same sense that humans do. If one examines the human institution of marriage for its true purposes, and how those purposes are accomplished in our society, one can see why we are not in need of such an institution. Caring for children, the most often defended rationale for marriage, does not need to be mandated, as all children are cared for by all, and enthusiastically. Compensating the dependent partner, usually the wife, who may give decades of devotion only to be cast aside for a younger model, is another rationale. In our society, no members need fear being rejected.
Maintenance is guaranteed, and at the same level for all.
Sexual ownership, the complete possession of the object, is seldom acknowledged as the reason for the marriage rites, but especially in highly conservative societies women are brokered openly. This has no parallel in our society, where ownership of the other does not exist. The institution of marriage is also relied upon in human society so responsibilities can be parsed out. Uncle Joe has nowhere to go, is now blind and deaf, so the family decides he should move in with his niece as she has married well and lives in a large house with lots of room. Her husband may protest, but as he is stuck in the marriage he will eventually give in to stop the guilt trips.