Monogamy?

There are multiple reasons species may engage in sexual relationships

-          Reproduction: Most animals have a drive to continue the species. For this purpose monogamy makes no sense. The male will mate with multiple females to have an offspring. The female will also mate with multiple males to increase probability of offspring. In terms of continuing the species, commitment, inheritance, parentage, don't matter. It is all about producing offspring. Monogamy actually reduces the chances of producing plenty of healthy offspring to propagate the species.

-          Pleasure: Humans, Apes, dolphins and some other species are known to engage in sexual relationships for pleasure. For this purpose monogamy or polygamy works. It is all about who or what gives people pleasure. Some people enjoy being with one person more, some people prefer promiscuous lifestyle.

-          Companionship: Humans, Turtles, Lobsters and many species are known to mate for life. Many species are social and tend to display social needs of long term companionship. Even amidst species with high reproductive drive, animals tend to build lifelong bonds. When a chosen companion animal dies, many animals go into mourning. Zookeepers find it frustrating as many will refuse to mate with others even if they are in a healthy mating age.

So it is not unnatural for a human to be monogamous or promiscuous. It simply depends on what their need/drive is at that point of time. When a human feels the need for committed companionship, they will settle down with one mate.  Until then humans tend to seek pleasure. While reproduction is still important and many human relations are built on reproduction, social evolution has made reproduction secondary to pleasure or companionship in humans. Many times all three factors play a part.

I don't think monogamy is cheating. However, situations like forced monogamy, unhappy monogamy, marrying only to seek pleasure or get an heir rather than companionship – could be considered as cheating oneself and others.

@King Anu – the only logical 'gamy' is mono and poly. Most others you list are sub forms of these, although some are downright absurd. But then again – Aishwariya Rai did engage in plantgamy, Sherlyn Chopra has relationships with vegetables – humans are a strange species after all.