Some religions teach that monogamy is the natural state of affairs, and that adultery is a sin because it is a violation of natural law. The debate about the benefits and drawbacks of multiple sexual relationships is not a simple one, and I would not want to be cornered into taking an absolute position that is meant to apply equally to every man and woman in every circumstance.
The one argument, however, that monogamists might have to abandon is the one about natural law. Genetic studies are showing that humans have no more been naturally monogamous during the last 150,000 years or so than have any other species.
A recent study of marmots even suggests that females often do select one mate by which they become pregnant and another mate to take care of her and her off spring. To make matters worse for the monogamists, offspring of these adulterous females are on average healthier and live longer than offspring from mothers who stick with a single male.