LUKE: It’s not biologically natural for people to mate for life. Animals don’t mate for life. Well, ducks do, but who the hell cares what ducks do?
In fact ducks don’t mate for life – most duck species, such as mallards, are monogamous, but for only for a single breeding season (a possible foreshadowing of the length of Max and Lorelai’s bond).
Some sea ducks are thought to return to the same partner when they make their way to the breeding grounds, but this is hardly the same as what a human marriage is generally like. In a few species of stiff-tailed duck, polygamy is the norm, with one male mating with several females.
Furthermore, even within pair-bonded duck couples, promiscuity is reasonably common – they are socially monogamous, not necessarily sexually monogamous.
The Annotated Gilmore Girls 