This is one of those seminal books (to me at least) that has a lot to say about the nature of human relationships.
p 36 - ...while there are various reasons why it could make Darwinian sense for a woman to mate with more than one man (maybe the first man was infertile, for example) there comes a time when having more sex just isn't worth the trouble. Better to get some rest or grab a bite to eat. For a man, unless he's really on the brink of collapse or starvation, that time never comes. Each new partner offers a very real chance to get more genes into the next generation - a much more valuable prospect, in the Darwinian calculus, than a nap or a meal. As the evolutionary psychologists martin Daly and Margo Wilson have succinctly put it: for males "there is always the possibility of doing better."
There is a sense in which a female can do better too, but it has to do with quality, not quantity. Giving birth to a child involves a huge commitment of time, not to mention energy and nature has put a low ceiling on how many such enterprises she can undertake. So each child, from her (genetic) point of view, is an extremely precious gene machine. Its ability to survive and then, in turn, produce its own young gene machines is of mammoth importance. It makes Darwinian sense, then, for a woman to be selective about the man who is going to help her build each gene machine.
whatever the ancestral environment was like, it wasn't much like the environment we're in now. We aren't designed to stand on crowded subway platforms, or to live in suburbs next door to people we never talk to, or to get hired and fired, or to watch the evening news. This disjunction between the contexts of our design and our lives is probably responsible for much psychopathology, as well as much suffering of a less dramatic sort.