“Why women stray: Evolutionary theory says men stray to increase offspring, but what motivates women? Enter the mate-switching hypothesis.” An excellent essay by David Buss and offered without much commentary. For men the important thing to know is that most women will stray if given the chance and the right opportunity. Be the guy she cheats with, not the one she cheats on.
I have a post aout David P. Barash’s book Out of Eden: The Surprising Consequences of Polygamy coming down the pike. I highly recommend it. Understanding the theory helps the practice. Most guys have neither.
Your girl’s male “friends?” Usually they’re alternate options.