For further insight into these issues, I found the below article very informative
www.nytimes.com/2011/07/03/magazine/infidelity-will-keep-us-together.html
Esther Perel's TED talk is also quite informative
www.ted.com/talks/esther_perel_rethinking_infidelity_a_talk_for_anyone_who_has_ever_loved
Evolutionary Psychology tries to deal with the why's of our behaviours. It's not an exact science and is far from perfect but there's a lot of insight and some of it has strong empirical support. Matt Ridley's The Red Queen is a good primer on the topic of sexuality from an EP point of view, the later chapters in particular deal with human sexuality in particular and why in general, men have a tendency towards promiscuity and why in general, women tend towards monogamy. Again, this is not perfect but a lot more insightful and grounded in reality then a lot of stuff that comes from the pulpits of this world