In medieval literature today, we talked in a very general manner about love, sex, and marriage in art. One interesting thing we realized is that even literature from polygamous societies does not have polygamous love stories. We have stories in which there is polygamy and conflict among wives, or stories in which a shrewish wife is a barrier to taking a new lover, or even stories (like David and Michel or Solomon and the Queen of Sheba) in which there is love between two lovers, but other lovers are curiously absent from the picture. This seemed a significant absence to me, but I'm not sure of its importance.
Some possible exceptions to the rule might be the Arthur-Guenevere-Lancelot triangle (though that becomes doomed as soon as it is made public), some of the stuff in the Tale of Genji (though, again, much of that seems to be behind closed doors), or maybe the Odyssey (in which the goddess appears to have a crush on Odysseus, yet helps him get home to his wife). Still, none of these seem quite right. Are there some obvious examples I am forgetting?