First humans may have been sexually monogamous (or at least almost), or may have come to be that way very early on, according to an interesting article in the 2014 September issue of Scientific American. This will be debated forever but I thought the article was interesting and presented opposing viewpoints as well.
Just the possibility that Org the Macho Caveman could have chosen sexual monogamy and survived quite well with it helps take another bit of pressure off men today who believe it goes against every fiber of their manhood and survival instincts to be sexually monogamous, therefore it's nearly unattainable -- and that science backs that up. Defeated by nature before even trying. Nature did also provide us with adaptability and flexibility in conditioning.