Is monogamy against nature?

Most creatures on earth procreate by selection. The selection of a mate is not a chance-event but a very careful elimination of the weakest. The male sealions fight their way and bleed to attract the female. The humming birds fall with a style and fiercely fight to protect their flower-rich territory to attract the female. In all these cases, fitness, health and age of the mate plays major role. Nature is amazing! It only wants the best of species to procreate. Nature in its own simple way reduces the chances of multiplication of bad genes. But what do you think of the humans? We with the power of our brain have created a system, which tries to protect the week and the bad genes.