Pairing is not only human nature, several other animals of lesser intelligence understand there is need for warmth and company as we go through this life. Wolves are known for their strong pack bonds and loyalty. Beavers are one of the mammals known to mate a life time. Other animals that pair for life include black vulture and Gibbons. Being single is not easy but it is a new dynamic that has become hugely popular among humans.
Well, males have generally been known to form weak sexual bonds because they enjoy their freedom to mate randomly. Some biologists argue in favour of the careless nature of men, that males seek to enhance their chance of procreation because their chance at true fatherhood is based on high risk probabilistic mathematics.