Although we like to assume our partners will always be faithful, most if not all people have had (or will have) more than one partner in their lifetime. This is also true for the most of the rest of the animal kingdom. Some species (such as birds) have social monogamous pairs, but different sexual partners. There are however, a small number of species that do actually stay faithful both sexually and socially to their partners for their whole lives. Therefore, I present what some people say is the most monogamous primate in the world the Owl Monkey!

This cute little creature has been shown to not only have socially monogamous pairs, but also genetically monogamous ones. To test this, researchers (Huck et al. 2014) collected samples from 128 individual monkeys living in 29 groups or as solitary individuals. They collaborated behavioral field observations with genetic samples from 35 infants born to 17 reproducing pairs and found that every infant was indeed the offspring of the reproducing pair. There was no evidence of extra-pair paternity. This is most likely due to the high level of involvement males have in the caring for their young. They play with them, feed them, and carry them on their backs. Without the high level of commitment from these proud papas, the intense faithfulness these guys show would not make sense (at least in the animal world). It even surprised the researchers by how faithful these small primates are and it seems to give them hope that monogamy (even in the animal world) may be useful and sometimes even preferred.
References
Huck, M., Fernandez-Duque, E., Babb, P. and Schurr, T. 2014. Correlates of genetic monogamy in socially monogamous mammals: insights from Azara's owl monkeys. Proceedings of the Royal Society B: Biological Sciences, 281(1782).