Dating people at the same time
As a result, it tends to be the exception rather than the rule that people report engaging in non-monogamy.
But again, forgetting how modern conventions have impacted this, have you ever heard of something called birth control?
Women who keep getting impregnated by guys who fail to stick around and help out with the kids may find themselves at an evolutionary disadvantage because they have to provide for themselves and their children entirely on their own (which probably proved exceptionally challenging in the days of hunter-gathers).
As a result, the best evolutionary strategy for women is usually to be selective and hold out for a partner who will be reliable.
However, because the act of producing a child is, undisputedly, simpler for men than it is for women, the sexes tend to approach the mating game very differently.
For men, the evolutionary strategy that makes the most sense is to sleep with as many women as possible because this maximizes their odds of having a lot of babies.