It's similar to wanting to play the violin. People can love good music and aspire to play the violin. But most find it difficult and won't practice. Monogamy, like the violin, takes practice and diligence because there are so many temptations. The authors contend that the sexes engage in infidelity for different reasons and that evidence for monogamy in human history is sketchy.
Males tend to be opportunistic and have sex out of marriage because it is available and pleasurable.
For women it is a way of obtaining something better than their mate, someone who may be richer, more handsome or more powerful, they said. In the western world, they said, monogamy only ascended in the late Middle Ages and the Industrial Age. The rich and powerful gained the cooperation of the masses by trading some of their wealth to low class men, enabling them to have wives and a stake in society. Kings still had many wives and concubines. The commandments would not have said 'do not covet thy neighbor's wife,' if people didn't covet her.