According to the authors, our attitudes towards sex and love are strongly influenced by our basic evolutionary history and biological needs. Men can't be sure that any given baby is theirs (the book says that studies show that about 10% of children born to married couples are not the husbands' biological children), so they are driven to have sex with as many women as possible to have a reasonable chance of fathering children. Women, on the other hand, know exactly which babies are theirs since they carried and delivered them. What they need is a man who will be willing to provide food, shelter, and security during and after pregnancy. The authors don't suggest that either attitude is right or wrong, although they do apologize to feminists for stating their thesis so plainly. They do say that by better understanding what drives the opposite sex, we can make our relationships better.
The book is written in an engaging and straightforward style, with very little scientific jargon, although it cites a large number of scientific and sociological studies of sexual behaviour and attitudes. A few cartoons and jokes are scattered throughout the book to lighten the mood a bit, but the underlying research is serious stuff.
Based on my own experience, I would say that most of what the authors say is credible and sensible. I recommend this book to anyone in a relationship or thinking of entering into one.