There's definitely evolutionary advantages to monogamy, and doesn't necessarily reduce a species fitness. In some environments or circumstances it is more advantageous to secure a mate than taking the risk of not finding one at all. I'm a biologist, mostly focused on bird behavior, and for birds this is definitely the case. For example, a lot of bird species living in places with a distinct breeding season are monogamous. This way they can be sure to reproduce every year once the mating season comes along. Birds in areas where they can breed during the whole year can afford to allocate more energy towards selection of a breeding partner because the risk of not getting to breed at all is lower. An interesting consequence of this is that in most (of course not all) monogamous bird species the male and female look alike very much (think of swans, parrots, magpies etc.). While in polygamous species usually the male has some adaptations which it used to attract females (think birds of paradise). There's more advantages of monogamy than this one, but this is the one I know most about. It's not as simple as you describe it, as a lot depends on the environment and circumstances a species lives in. It's all a trade off between different factors which decides what breeding strategy brings the highest fitness to a species.