Freedom of expression is a very great good and so justifies allowing considerable (though not unlimited) psychological pollution. There may not be such a potent compensating good in the case of physiological pollution. It may be that a policy of additional legal restriction on psychological pollution would carry a greater moral cost than does the current legal restrictions on physiological pollution.
Surely anything that promotes sexism is, to the degree and for that reason, a bad thing. Truth is, the popular media and advertising reinforce all kinds of biases and prejudices (against older people, against people who do not fit social standards of beauty or attractiveness, against poor people, against people of color--by inadequate representation, and so on and so on). The media make money from doing so, because people have the interests they have--and these interests are often sexist and biased in all of the relevant ways.
So you want to ban all of the ways in which the media promote or reinforce such wrongs? Well...you will have a lot of censorship to do!
On the other hand, as Pogge suggests, surely there are more important concerns (in regard to sexism specifically, and in regard to making the world a better place more generally) than becoming overly concerned that Page 3 shows the breasts of young women. Before you get too far gone in moral indignation about this issue, it might be wise to consider whether there is anything else more important in the world going on--for which some effort from you would make a difference. Spending lots of moral capital on relatively minor evils, and thus ignoring more important ones, seems to me to be a significant lack of moral judgment in itself.
I'm not saying that sexism isn't bad--it is bad. I am saying that you probably have more important things to attend to than the Page 3 issue... If you are looking for trouble in the world, it isn't hard to find. Use your best judgment as to which of the endless troubles you find are most worth your concern.
I associate recent defenses of this claim with criticisms of "sex positive feminism," which stresses ways that embracing and affirming their their own sexualities can help feminists to resist the patriarchy and can empower themselves and others; the basic criticism by MacKinnon and others is that the immoral consequences of patriarchy are so intense and pervasive in our culture that they undermine this sort of sexual liberation.
So, for example, in some articles Catherine MacKinnon's position is stronger and more radical than the one Alan describes because her pessimistic view extends to all expressions of sexuality, and is not limited to heterosexuality. In "Sexuality, Pornography, and Method: 'Pleasure Under Patriarchy'", MacKinnon argues that there exists in our patriarchal society a "rape culture" that is so strong that it is internalized even by those who oppose the patriarchy most strongly. Her pessimistic conclusion is that this makes morally problematical all sexual activity, including for example masturbation and homosexual sexual activity. Thus her article ends with this haunting conclusion: "I do not know any feminist worthy of that name who, if forced to choose between freedom and sex, would choose sex. She'd choose freedom every time." (That conclusion is a quote from Ti-Grace Atkinson's article "Why I'm Against S/M Liberation.")
I'm not completely sure what you are asking. Presumably you do not want to know what it is that feminists know that others don't, though I could write you a book on that. I suspect what you're curious about is feminist theories of knowledge, or feminist epistemology. This is a book-length topic, too, but I'll try to say enough in a short space to give you an idea what is going on.
Feminist epistemology -- really, feminist philosophy generally -- begins with a simple observation: virtually the entire body of our received philosophical thought has been developed by men, and by socially privileged men at that. The question then arises whether this homogeneity among philosophers has resulted in some kind of bias or distortion in the theories produced. In philosophy, suspicion is heightened by the fact that our methodology relies heavily on "intuitions" that theorists presume are universally shared. What if they're not? (And by the way, there's excellent evidence, apart from evidence adduced by feminists, that some key philosophical intuitions -- judgments that form the basis of very influential philosophical theories -- are not universally shared. See, for example, question 13 and discussion: http://www.amherst.edu/askphilosophers/question/13 . Also, philosophers Shaun Nichols, Stephen Stich and Jonathan Weinberg have found cross-cultural variation in intuitions about knowledge -- you can access their paper at http://www.rci.rutgers.edu/~stich/Publications/Papers/Meta-skepticism.pdf )
Also, philosophers have tended to take particular kinds of knowledge as paradigm in their theorizing and to neglect others – theoretical rather than practical knowledge, scientific rather everyday knowledge. This pattern of focus might reflect both the gender and social locations of the theorizers, since it is only privileged individuals that have, historically, had the leisure and resources to engage in academic or scientific pursuits. How might knowledge have been conceived if theorizing had taken into account the kinds of knowledge possessed by women and by social subordinates – the kind of knowledge implicated in interpersonal relations and in practical engagement with the material world, "common sense" and "folkways"? Finally, the theoretical aims of much traditional epistemology have not been oriented toward particular philosophical questions that are of pressing concern to feminists, theoretical questions about the role of social relations in the construction of knowledge, and practical questions about the processes that confer epistemic expertise, and about the (still) low rate of involvement by women in academic and scientific inquiry, especially in philosophy.
Feminist philosophers differ in their answers to these questions. There is a broad consensus (of which I, myself, am not part) that holds that the way in which the knowing subject has been conceived within traditional epistemology embodies a "masculinist" perspective, specifically in its abstract, individualistic character. ("Masculinist" means that the perspective in question is not just that of any man, but rather of a man theorizing in the context of a patriarchal social organization.) Many feminist philosophers (again, not me) also believe that fundamental epistemological concepts, like "objectivity" and "rationality," have been conceived in ways that make it virtually contradictory to attribute these traits to women. (And many prominent philosophers, including Aristotle and Kant, explicitly denied that women had reason in the same sense as men did.) Some feminist philosophers (I’m in this camp) hold that traditional concepts and theories are not marred by masculinist bias, but that epistemology still needs to be re-oriented toward issues raised by feminist theory and practice, and that new insights about knowledge in general will arise as a result.
Whatever they believe is the outcome of the critical project of scrutinizing traditional epistemology for masculinist bias, most feminist epistemologists today have moved beyond that to more constructive epistemological projects. Some are trying to define "successor" notions of concepts like "objectivity" – notions that, according to these theorists, more adequately capture the perspectives and experience of all persons. Many are developing accounts of knowledge that make different assumptions about the knowing subject than the ones informing mainstream epistemology, and in particular, accounts that assume that knowledge is essentially social. Some are applying traditional philosophical methods or mainstream epistemological theories to neglected epistemological questions.
For more detail about the content of either the feminist critiques of mainstream epistemology, or about the constructive projects in feminist epistemology, a good place to start is Elizabeth Anderson’s entry on "Feminist Epistemology" in the Stanford online Encyclopedia of Philosophy: http://plato.stanford.edu/entries/feminism-epistemology/
I don't think that there's any special reason to think that female philosophers are more insightful than male philosophers -- or vice versa, for that matter.
Nonetheless, it may be true that female philosophers on occasion have different insights from their male counterparts. For example, feminist critiques of traditional ethical theories (like those offered by Kant and Mill) suggest that those theories focus on masculine values and ignore values that are of fundamental importance to women. Feminist philosophers have worked to develop an "ethics of care" that looks at moral questions very differently from traditional theories.