What are the reasons behind monogamy in humans?
First off, I think monogamy is a socially and culturally determined construct in man. 

In the animal world both monogamy and polygamy are encountered. Monogamy increases the chance of the survival of individual offspring, as dad can help raising the offspring  and can defend the family. This increases fitness.

On the on the other hand, polygamy increases the spread of the (male) genes and hence increases the chance that the male gene pool is spread. 

The relative benefits of both strategies depends on many factors, including resource availability, offspring need, parental care, and mating dynamics (Klug, 2018). 

Reference
- Klug, Front Ecol Evol (2018); 2018.00030