In choosing partners, women face one great tradeoff. They want both quality and loyalty. Ideally you hope to get some combination of both. But highly desirable partners always have other options. Losers are probably less likely to stray, but women don’t want to be with losers if they don’t have to be. Individual women seem to differ a great deal in how much they value quality versus loyalty, with some willing to chase around a celebrity in hopes of spending one night with him and others seeking to capture one mediocre guy who will provide long-term stability. Most are somewhere in between, and managing the quality-loyalty tradeoff is one thing that makes finding and maintaining relationships difficult.
Men of course can all promise to be loyal. But how can she know? In our environment of evolutionary adaptedness, once a woman got pregnant and had children, she became vulnerable at the same time her value as a mate decreased. Just as importantly, every day Father Time is chipping away. Men’s options may narrow as they get older too, but the effect is nowhere near as extreme. If a woman partners up with a rich and powerful man, or one who has the potential to become rich and powerful, she has to worry that when her attractiveness declines and his mate value is still high, the temptation to stray will be particularly strong. See Donald Trump, who has traded down for a younger wife twice and might’ve done it a third time by now if he hadn’t become occupied running for and serving as president.