Human beings are not neurologically wired to be monogamous.
If you think about a human brain being two brains, one 'reptillian' and ancient, and the other (the neocortex) as being modern and sophisticated.
When humans were very young, multiple partners are what kept the species going instead of dying out. And men are not the only ones who want more than one partner: science tells us that when the human race was young, females mated with many males. Current statistics show little difference between the male to female 'cheating' rate.
At some point, more secure, long term pair bonding formed along with social elements, language, and spirituality (the development of the neocortex).
Choosing monogamy takes a personal ethic, morality, spirituality of pair bonding, and, a high degree of self discipline.