On the other side of the coin, what sorts of species have multiple partners, and for what sorts of reasons?

A Chimpanzee female will have sex with multiple partners to ensure that her offspring isn't killed. Male chimpanzees will kill offspring that isn't their own. This confuses the male chimpanzees and they will leave the offspring alone.

In a few primates, a new Alpha male will kill, or try to kill, any pregnant female, since the child isn't his. He then tries to impregnate all the other females. Doing so is his way of insuring that his genes, which are now the best in the tribe, are the genes of the tribe's future. Think of it as an enforced Darwinism. Some scientists (and some men in general) have postulated that this drive, trying to spread his individual genetic code, accounts for an apparent tendency of the human male to "roam."

"Honest Honey, I just couldn't help myself. I did it for our future. All of our future."

(Hey folks, I didn't make that conclusion, so leave me out of the criticism.)