nonplussed, your post has given me food for some thought.
Perhaps what you meant to say is that these things may be true in stereotypical or perhaps normative average terms. And yes, that distinction matters- a lot.
Partners with families are still individuals with their own need for self-fulfillment, aspirations and interests. Sometimes these needs come ahead of the seeming "best outcome for the family unit", or even ahead of the relationship itself.
Many men and women who are married choose not to have children or cannot have them biologically and are unwilling to adopt. Many men and women remain single. All such people resent having their employers make decisions about their pay, retention or advancement based on assumptions about whether or not they're likely to have a family in future and whether or not that will reduce their level of commitment and availability to their jobs...and, regrettably, human nature being what it is, normative stereotypes DO inform such decisions on the part of managers and employers even when they try to set aside such assumptions and bias. That's a real problem, leading to real experiences of unfair treatment particularly by women, and it's a very tough one to solve.
Not everyone meets their spouse before either have chosen a career. The age of 1st marriage and the woman's age at the birth of her first child have both risen a lot over the past few decades. It is true that a partner's career choice can change, or the choice can be taken away or altered by circumstance. It is also true that even very well educated, analytical and rational people can completely throw these attributes to the wind when choosing a spouse- and though that can sometimes lead to a brilliantly successful outcome, it can also lead to years of misery or a complete disaster.
"Power balance" is something that all relationships need to manage and negotiate- that goes for economic power, decision-making power and all sorts of other sorts of "power" in a relationship. Couples who are well matched, mature and not mentally ill can have successful and long-lasting relationships despite a power imbalance in either direction, or with a very equal power balance, or a power balance which varies at times throughout the relationship. A equal power balance in the most trivial sense, i.e. where both spouses have jobs of similar responsibility, social prestige and income potential, is not necessarily a good predictor of a successful or unsuccessful partnership or marriage. A very unequal power balance can be an ongoing source of stress even for people who are otherwise very well matched.