First of all, the most insightful description of hypergamy:
“Men may have a tendency to seek sexual variety, but women have simple tastes in the manner of Oscar Wilde: They are always satisfied with the best.” F. Roger Devlin – Sexual Utopia in Power
What it means in layman’s terms is that women place a much heavier emphasis than men on mating only with the best possible partner. Men are (more than) willing to have sex with a wide variety of partners but women want only the best. For example if an average fellow is presented with a hundred gals he’d probably happily have sex with 90 of them, but if you present a hundred guys to an average girl she won’t touch any of them but the top 5. So, women are hypergamous but men are not.
Another important thought:
“In marriage, a man and women contract to forsake all others. Now ask yourself: If either were going to defect from this agreement, what form would the defection take? Men, with a polygamous objective function, typically defect by trying to find an additional woman. Women, with a hypergamous objective function, betray by trying to replace their existing man with a better one.”
Now, there’s nothing inherently wrong with hypergamy because it’s a biological drive and not a character fault. People can’t be blamed because of their instincts. Basic biological impulses – be them god-given or evolutionally refined – are not the subject of moral judgement.
Still most men view hypergamy as something nasty or evil, and so they tend to view women as nasty and evil too. There are two main reasons for this. The first one is the quote above: polygamous male nature doesn’t destroy marriages like hypergamy does. It’s only natural that most men – and any human being with a sense of fairness for that matter – find it revolting if an otherwise perfectly fine man is kicked out of his home and banned from the life of his children for nothing more than his wife finding an other man with a higher sex rank. The other reason is that men’s instincts are a lot more “straightforward”, meaning that women’s sexual market value (sex rank) depends entirely on her own personal characteristics while a man’s sex rank depends on other factors not inherent to his personality, like wealth, social status and such. This results in the sex rank of a guy being largely unrelated to his real personal qualities. (Hence the proverbial “nice guys finish last”.) Most men find this superficial and immoral even if they understand that this is not a conscious choice by women.