When men outnumber women, women are valued. When women outnumber men, women are willing to make big compromises in their pursuit of men. In Siberia, there is such a shortfall of men that there is a grassroots campaign to legalize polygamy.
And, it isn't just society wide gender ratios that matter. What matters are the gender ratios in particular pools of people who see each other as marriagable.
To that end, "Whether the sexual double standard is cultural or biological, it’s finding traction in the increasingly lopsided sexual marketplace that is the American college campus, where women outnumber men, 57 percent to 43 percent." In contrast, people with only a high school education (or less) are more often men.
This goes a long way towards explaining the disparate marriage trends in the two groups, with marriages of college graduates lasting longer and those of high school educated men becoming more fragile. A college educated woman lucky enough to marry a college educated man may have a hard time replacing him with another college educated man. A high school educated woman married to a high school educated man may have a much easier time replacing him with another man with at least as much of an education. Many women, as a result, have given up on the "traditional" economically dependent woman making sacrifices for her man in exchange for loyal support model entirely.
Then, in the extreme of this trend which has been prophet in the past, there is the state of black marriage.
70 percent of black women are unmarried, and they are more than twice as likely as white women to remain that way. Those black women who do marry are more likely than any other group of women to “marry down.” This is often chalked up to high incarceration rates—in 2009, of the nearly 1.5 million men in prison, 39 percent were black—but it’s more than that. Across all income levels, black men have dropped far behind black women professionally and educationally; women with college degrees outnumber men 2-to-1. In August, the unemployment rate among black men age 20 or older exceeded 17 percent.
There had been no racial disparity in marriage rates in 1950, when 64% of black women were married.
(As an aside, the dramatic change is Exhibit A in the case that Bell Curve style population genetic disparities, or "Roots" style ancient cultural legacies, aren't a good fit to explaining this change. Black women and black men have virtually the same genetic endowment, and the change in the nature of black marriage largely followed the Civil Right Movement from a status quo ante that wasn't racialized.)
But, the bottom line, is that it is all very complicated. Theory gets you only so far. You have to know a lot of nitty gritty detail of the situation that people encounter in their lives to make sense of what is happening. Even if you do know, that doesn't mean that the best social response to these trends is at all obvious.
One footnote: the "missing women" issue in many developing countries may have a lot more to do with disease mortality rate transitions, and a lot less to do with selective abortion, infanticide and short changing of young girls in families than we is conventionally believed.