Im reading a bunch of classic books this summer just to learn some new shit. This one book called Sperm Wars will blow your f@#$ing mind. It explains exactly why women and men cheat, uses about 40-50 situations then exlpains in detail biologically why they cheated, not psychologically.
Baker (the author) says everything men and women do is behavoir that emulates the sperm and egg. Some crazy quick facts about sperm.

*Some sperm have 2 heads, 3 heads, and 4 at the most.
*Young sperm (newly produced) are the killers (kill other mens sperm, 20%) as they age their role changes to the egg-getters (20%) than the blockers (60%).
*If you take different sperm from 2 men and mix it under a miscroscope, than add a drop or 2 of cervical fluid from a female, her fluid sends a chemical message to the sperm that causes them to fight and kill each other while searching for the egg. (But in that case will die and never find it)
*All sperm contain a capful of lethal poison in thier heads which can kill 4-5 other sperm, the older sperm commonly engage in suicide killings.
*Evolution as a species relies on "smart vaginal muscles" inside a women and thier ability to use cervical fluid to filter out weak sperm. A womans body will actually encourage healthier sperm (better DNA) and will make the weaker sperms job harder (during flowback, all women know what that is, hopefully men too)
*Men masturbate between regular intercourse for one main reason. His job as a male is to be a reproductive success. So by frequent (or not so frequent, get to this later) ejaculation, he sheds himself of the weak old sperm. A sperm onece releases only has 4-5 days to find the egg before death. Too many old sperm clog up the cervical fluid and prevent younger egg-getters from finding the egg. Masturbation = younger sperm = more chance of fertility (whether or not the man wants to impregnate the woman)
*Frequency of masturbation is dependent upon the actual size of a mans testies, as men age testies shrink and so does sperm production. Men with bigger testies produce more sperm and have more of a need to shed the stress of more frequent production through masterbation.
*A man's job to = reproductive success, is to fertilize as many females as possible. A womans job to = reproductive success, is to pick the healthiest sperm with the best DNA. Both have fairly rational reasons to cheat from a biological standpoint.
*A womans job is to hide her fertility from a man (and herself). In less sophisticated species that dont hide fertility, like monkeys, the men will often fight, mame or rape each other or the female to fertilize her. Human females to constantly evolve (which requires not ro die) are programmed with certain mechanisms to hide fertility from the male aka PMS (which not all women get but explained in book) PMS happens when women are in thier most fertile state and her mood swings are natures way of pretecting her from a male who without the mask of PMS would get highly aggressive with her. Depends on a females liklihood to demonstrate fertility and her need to cover it up.
*A study was done where men sniffed tampons in a test tube at different weeks of a womans cycle. All men said the smell was most pleasant at the time the women happened to be most fertile, and had the worst smell after releasing her most recent egg.
*(this one blew my f*&&*kn mind) Back in the day as primates, yes we use to engage in oral sex, but not for pleasure. "Nuzzling the vulva" even today although we have developed certain areas of our brain to "appreciate" such activity that we didnt posses thousands of years ago serves much deeper functions. When we perform oral sex, subconciously our minds are gathering data, 3 main things. 1) Is this women diseased (disease smells and prevents our chance of reproductive success) 2) Has she recently been fertilized by any other males (which instantly causes men to want to inseminate the female) A lot of rapes are commited after the instance a man discovers his womens infedility. 3) How fertile she is, its been documented sweeter smell always = more subconcious incentive to engage in actual intercourse.

Again all this is governed by subconcious behavoir. Whether or not you disagree with me, read the book and see for yourself. This guy turned age old biological assumptions on thier head with this book.

So if anyone likes reading classics, this book is a must.