The modern ''courtesan'' that prompted the remarks was Zahia Dehar: now a luxury lingerie designer mentored by Lagerfeld, previously a teenage ''escort''. Algerian-born Dehar insists that she had no pimp, and was free to have sex with whomever she wanted. But the scandal rightly remains: when Dehar was earning four-figure sums from prostitution, she was 16, which is illegal in France.

To be clear, the problem is not teenage sexuality; it is ignorant and counterproductive to pretend young women are not sexual creatures, like the rest of us. The problem is not sex work, which, when properly regulated and managed, can presumably be a profession like any other: unionised, skilled, stressful, tiring, rewarding and so on.

The problem is power: minors paid for sex by adults, and the dangers of physical and emotional damage that come with this. It takes a great deal of maturity to negotiate sexual relationships, even as adults in committed relationships. How much more psychological development is required to deal with the commodification of sex, when one is not yet physically or emotionally mature? Add to this poverty, abuse, and social isolation - common to many young sex workers - and the threats to well-being are obvious. Let me be blunt: for all her beauty or charm, Dehar was just a kid.

Which brings us back to Lagerfeld, and his explanation of Dehar's allure.

By using nationalism, he adds patriotic polish to exploitation. Robbed of its Gallic-pride sloganeering, his argument runs like this: ''Under-age prostitution was common in the past in this country, and other countries did it too.'' We might say the same of European fascism and Islamic fundamentalism. Are they reasons for pride? Not for the victims. The ethical question is not ''Is it popular?'' but ''Is it just?'' And we have laws against paid sex with minors for a good reason: it is damaging.

That Lagerfeld's argument is dubious is to be expected. It is not really supposed to defend teenage prostitution. Instead, it shifts the goalposts from gender equity and social harm to romance, style, glamour, and France before post-colonial complications. It is, in other words, a defence of his brand.

When it comes to gender violence or exploitation, the media is full of these pseudo-explanations. Take the recent characterisation, by British politician George Galloway, of sex without consent as ''really bad manners''. As Karen Pickering recently noted on the ABC, Galloway was not genuinely interested in analysing sexual assault. Instead, he was defending Julian Assange. Rape was an issue to be deftly avoided, as one steps around blood outside a nightclub; it suggests some violence, but best to keen one's shoes clean and keep walking.

More fundamentally, the point is exactly what modern feminists have been arguing for over a century: women are, in the minds of many, not quite rational, free agents, with dominion over their own bodies and minds. They do not deserve the same recognition of autonomy and complexity. It seems absurd to have to write this; but many absurdities acquire longevity by becoming the status quo.

If a young woman is raped while helplessly drunk, for example, one defence is that she somehow ''asked for it'' because of her clothes, makeup, body language, or previous relationships. In other words, she is free enough to be given responsibility for the crime, but not free enough for her autonomy to matter. Yet we do not make this argument for assault victims: if a drunk man is kicked in the face while passed out on the footpath, we don't say he ''asked for it'', despite the fact that he was intoxicated in a violent street. His victimhood is straightforward: his body was violated, and his attacker is vilified. Violations of women's autonomy do not receive the same simple condemnation.

Likewise, the problem with teenage escorts is not that they are having sex, assuming they are mature enough to consent. The problem is that, in many cases, they are having sex for money, because that is the only way they are valued: as sexual pleasure for older, richer men. Not all sex work is a tragedy of forced choice. But, for many young women, choice is a luxury for their clients. And these power relations, despite their easy familiarity, are in no way ''natural''. This is presumably why Dehar is now a designer: because she hopes to be something more; to have more choices, opportunities, skills.

There are many fantasies being sold by Lagerfeld: beauty, power, money, sex. Imagine another, a cloud-cuckoo-land in which women had equal independence, ambition and opportunity. Now that, Karl, would be a tradition to admire and copy.

Dr Damon Young is a philosopher and author. His next book, Philosophy in the Garden, will be published in December 2012.