Last month, a teenage boy in Iowa made headlines when he forfeited a high school wrestling match rather than face his opponent – a girl named Cassy Herkelman. Joel Northrup claimed that "as a matter of conscience and faith", he couldn't engage in physical contact with a female wrestler (in Iowa as in many US states, girls and boys do wrestle each other and have for some 20 years). Northrup immediately became a hero to American social conservatives, who anointed him a young icon of traditional masculine chivalry.

But the larger symbolism of Northrup's act has gone unnoticed: this wasn't about gallantry, or a fear of sexualised contact with a girl. Though fear of losing to a young woman was surely a factor, there's a bigger story here: the longing of too many young men for all-male spaces, in which they don't have to compete with women as equals.

Northrup's forfeit came the same week as the release of a new book, Manning Up: How the Rise of Women Has Turned Men into Boys. Penned by Kay Hymowitz, a prominent American conservative, Manning Up argues that women's achievements have emasculated men, leaving teen and twentysomething lads with precious little reason to strive for anything more than a life of puerile amusement. For Hymowitz, the feminist movement's success in integrating into traditional male spheres has robbed men of a chance to feel important and valued. As a consequence more and more would be choosing to opt out, retreating to pot, pornography and playing video games on their mothers' couches.

Though some have suggested that Hymowitz oversells the problem of young men's irresponsibility, the real problem with her thesis is her claims about its cause. If more and more young men are drifting aimlessly through their 20s and 30s and turning adolescence into a quarter-century project, the problem isn't women: it's the archaic but still potent rules of masculinity. These "guy rules" or "man laws" define manliness as the absence of anything feminine: the first rule is "no sissy stuff." What is masculine are only those things that women can't do, like write their names in the snow while urinating – or until recently fly fighter jets and wrestle boys.

Even within their short lifetimes, Northrup and Herkelman have seen women move into more and more positions that were once exclusively male. For lads whose sense of masculinity is rooted in doing what women cannot do, this means retreating to the few spaces where women are still not permitted to enter. As soon as a girl like Cassy does intrude suggesting that she has the right and ability to compete at a once all-male activity, that activity instantly loses its masculine cachet. And so millions of young men on both sides of the Atlantic are doing what Joel Northrup did: forfeiting the competition altogether.

Contrary to what Hymowitz argues, male responsibility does not require female vulnerability to thrive. The problem, rather, is a culturally constructed one: men have been raised to believe that many of the things truly worth doing are those that women cannot do (or more accurately, were not allowed to partake in). Despite his claims about his faith, Joel Northrup didn't forfeit his match because of anything in scripture forbidding boys and girls from wrestling (there isn't). He didn't forfeit because of chivalry. He didn't even forfeit because of fear of losing to a girl. He did so because the very presence of a female competitor robbed the sport of its power.

Sadly, Northrup is not the only young man who would pick up his proverbial toys and go home rather than compete in an egalitarian environment. And make no mistake: it's not women's fault for wanting to compete with men, whether on wrestling mats or corporate boardrooms. It's the narrow definition of masculinity that requires the exclusion of women that is to blame for this crisis.