I’ve never tried being a man, but the writer Norah Vincent did in a year-long experiment for her book Self-Made Man, and she found out two things. Firstly, that people were amazingly eager to accept her as a man on the basis of a bound chest, a flat-top haircut, masculine clothing and some ersatz stubble. Secondly, that while it was easy to get classed as a man, living in that class meant being subject to constant scrutiny: “Someone is always evaluating your manhood [...] everybody is always on the lookout for your weakness or your inadequacy”. In the end, Vincent suffered what she calls a “crack-up”, attributing it to the pressure of her restrictive alter-ego.

The best way to think about gender is as a kind of hell. Men occupy the narrow centre, with various degrees of “non-men” expanding outward in concentric circles, every region bristling with demons ready to prod deviants back into line or cast recalcitrants into the outer darkness. A man who falls out of manliness can only fall so far. A woman who fails at femininity, as Glosswitch describes, has failed doubly by gender’s underworld logic: first of all to be male, and secondly to be a woman, a low enough condition on its own even before you get banished to the far fringes of the inferno.

In the last few weeks, comments from Woman’s Hour presenter Jenni Murray (“Be trans, be proud — but don’t call yourself a ‘real woman’”) and novelist Chimamanda Ngozi Adichie (“transwomen are transwomen”) have led to furious debates about what a “real woman” actually is. What is not discussed is what a “real man” is and why particular male people might not meet the criteria. This is frustratingly obvious once you’ve noticed it, and yet it largely does pass unnoticed, because this is how gender works: a man is the thing that never has to ask what he is, because everything out in the emasculated penumbra is defined with reference to him.

For an extraordinarily blatant example of this, see Saira Khan’s Mirror column responding to Murray. Khan has a transwoman friend, she writes, who has “now found love with an amazing man who wants to marry her and doesn’t care about her past. And if he accepts her as a real woman, where does Dame Jenni get off thinking she can tell her she isn’t one?” It’s an appeal to the authority of the penis that has the curious effect of defining a “woman” as “whatever a man wants to have sex with”. He is the subject, she is the object. And according to this argument, a male sufficiently objectified becomes by definition a she.

A man can fail at masculinity and fall out of the rank of the manly in many ways. The Ancient Greeks thought a small penis implied the desirably masculine quality of restraint, hence the adorable mini-wieners on their idealised statues of men (the corollary stereotype of well-hung animalistic hypersexuality is alive and well in modern racist porn on the “big black cock” theme). Twentieth-century anti-Semitism portrayed Jewish men as dangerous precisely because they were allegedly unmanly: the Blood Libel is linked to a belief that Jewish men were so womanly, they menstruated, and needed to replenish their circulation. Byron, and other upper-class critics, dismissed Keats as a “Cockney”, a word which implied both low breeding and effeminacy.

The rules of being a man are arbitrary, racialised and entwined with class. And men defend them, even when the rules are used against them. Being teased over his masculinity did nothing to make Keats sympathetic to women: in an 1817 letter, he described himself to be “vexed and teased by a set of Devils, whom I detest so much that I almost hunger after an acherontic promotion to a Torturer”. These “Devils” were women writers. Life in the middle of hell is tightly packed and overheated, with little room to move; but nevertheless, it’s better to be the masculine One than the non-man Other. Satan might be the most damned, but he’s still in charge.

Masculinity is rigid and impenetrable, obsessively exiling its impure elements. Critics of Murray and Adichie have accused them of “biological essentialism” and attempting to “restrict womanhood”. There is no reciprocal accusation against men for violently excluding feminine males from manhood, because manhood is supposed to be exclusive, while womanhood should be (in the memorable words of Maggie Nelson’s The Argonauts) “like the blob that ate Detroit” – able to accommodate anything that masculinity rejects. In our gender revolutionary times, people still seem to find it easier to accept the existence of a “female penis” than of little boys who like princesses and pink.

If we understood that a “real man” can be any human male, and that human maleness encompasses a whole range of personalities, aptitudes and preferences – if we could do that, then men would be free from their narrow circle of gender hell. But if that happened, the hell of gender would collapse entirely, and men would lose their position of dominance. The demand that womanhood be unbounded is really a demand that male authority be unquestioned. So ask the question: not what is a woman, but what is a man? Who gets to decide? And why is it so important that the “un-manly” be bundled elsewhere?