It's not just because women are, on average, shorter than men.

Men are bigger and stronger than women. That generalization, although true, doesn't adequately describe how sex affects our modern lives. In the first place, men's and women's size and strength are distributions. Strong women are stronger than weak men, so sex doesn't tell you all you need to know. Otherwise, as retired colonel Martha McSally put it with regard to the ban on women in combat positions, "Pee Wee Herman is OK to be in combat but Serena and Venus Williams are not going to meet the standard."

Second, how we handle that average difference is a matter of social construction: We can ignore it, minimize it, or exaggerate it. In the realm of love and marriage, we so far have chosen exaggeration.

Consider height. The height difference between men and women in the U.S. is about 6 inches on average. But Michael J. Fox, at five feet, five inches, is shorter than almost half of all U.S. women today. On the other hand, at five-foot-ten, Michelle Obama is taller than half of American men. So how do people match up romantically, and why does it matter?

Because everyone knows men are taller on average, straight couples in which the man is shorter raise a problem of gender performance. That is, the man might not be seen as a real man, the woman as a real woman, if they don't (together) display the normal pattern. To prevent this embarrassment, some couples in which the wife is taller might choose to be photographed with the man standing on a step behind the woman, or they might have their wedding celebrated with a commemorative stamp showing her practically on her knees—as the British royals did with Charles and Diana, who were both the same height: five foot ten.

But the safer bet is just to match up according to the height norm. A new study from Britain—which I learned of from the blogger Neuroskeptic—measured the height of the parents of about 19,000 babies born in 2000. They found that the woman was taller in 4.1 percent of cases. Then they compared the couples in the data to the pattern found if you scrambled up those same men and women and matched them together at random. In that random set, the woman was taller in 6.5 percent of cases. That means couples are more often man-taller, woman-shorter than would be expected by chance. Is that a big difference? I can explain.