In a lek based mating system, males and females regularly gather at some location and the males put on a display. The females inspect the displays, then, after much wandering about, have sex with one of the males, never to meet that male again.
Absent patriarchy, humans predominantly mate on the lek system.
In monogamy and polygyny children have natural fathers. In lekking, the don’t. Today, most children do not live with their father. Therefore, lekking is the today predominant system, not monogamy, not serial monogamy, not polygyny.
In general, a species that leks will obviously produce fewer offpring than a monogamous species, about half as many. Hence the fall in fertility.
In the general span of history surveyed by Udwin in “Sex and Culture”, societies that reproduce by lekking disappear, while monogamous societies expand and overwhelm their neighbors.