In other words, a heterosexual marriage helps a man's career thrive. For women, it means more work and less pay, or the financially tenuous position of staying home full-time and hoping your marriage (and only source of income) lasts.
Women today expect more egalitarian relationships than they did a generation ago; and while men are more pro-feminist than ever before, plenty haven't caught up. (My advice to women is "Don't marry a man who doesn't pull his own weight.") For the many women married to sexist men, a little money can ease the disputes: hiring a nanny or a housekeeper, for example, doesn't just mean a clean house, it lets the couple gloss over the assignation of women to the domestic sphere. It helps them believe the relationship is relatively equal.
The nanny or the housekeeper doesn't usually have that option, and may simply prefer independence to marriage with someone she needs to clean up after. Outdated notions of masculinity (like the man as breadwinner) also prevent many working-class men from seeing themselves as marriage material.
For women for whom college isn't a possibility or was never on their radar – a reality for many Americans – there isn't the same incentive to wait until they're 30 to get married and have children. Years in a low-income job won't make life more stable, and might lead to only a marginally higher income. Marrying a partner with worse prospects will cause greater instability, if not ruin. But having a baby, even while young and single, can bring social status, love, and attention – not to mention temporary help with things like health insurance and housing.
If you're highly-educated, financially stable, and marrying later in life, your marriage likely has few roadblocks. If you're not – if you're part of the demographic that many conservatives say has "fled" marriage – staying single may actually be the rational choice.
The problem, then, isn't that many Americans aren't getting married. It's that too many Americans are constrained by outdated gender roles and financial insecurity.
So why not advocate for policies that promote prosperity and happiness regardless of marital status? Why not try to put more Americans in the same economic position as the people whose marriages are thriving?
Because those policies are liberal. And because they don't involve shaming single moms or poor people.
If the marriage advocates calling for a "new conversation" actually want progress, let's do what works. Strengthen the social safety net so that a layoff or a pay cut doesn't push any family over the cliff. Require equal pay for equal work, so that women can give as much as they've earned to their household. Reform traditional gender roles so that a man's value isn't just his paycheck, and a woman's value isn't trapped in the house. And support birth control and abortion access, so that women can prevent the unwanted pregnancies that so often upset financial and relationship stability.
Most importantly, end the cultural mythology around marriage.
Marriage is great, if you want it – but it's best begun out of love, not societal pressure. We should care about the security and happiness of our neighbors regardless of their marital status. The decline in marriage rates for low- and middle-income Americans is alarming not because marriage should be a universal goal, but because it reflects the current state of marriage as a luxury good, a way for the upper classes to perpetuate their wealth and power.
The answer isn't "Get married and you'll be rich and powerful." The answer is to break down the extreme inequalities that incentivize marriage for some and make it seem out of reach for others. Marriage should simply be one model among many for human kinship and a strong family. Let people do what they want.