I see pornographic images in mainstream spaces and I never give it much thought. Maybe it's a copy of the Sunday Sport next to an offer on Maltesers, with a picture of a mostly naked woman in an auditioning-for-a-porn-film pose; or a T-shirt in the window of a high-street chain, with a picture of two naked women snogging; or a van going past with a picture of a pneumatic-breasted, inexplicably naked woman painted on to the cab. They're such cliches, so pappy and unoriginal, that they've always washed over me, like the sound of Justin Bieber or someone reading the lottery numbers. However, in the past I've always thought that, because they made no dent on my consciousness, they didn't matter. What should have been obvious is that, like air pollution, just because you can't always see it, doesn't mean it's not poisonous.

Mascara on sale in Debenhams in York. Photograph: misspolly1 via GuardianWitness

In response to a GuardianWitness callout, Karla Willows sent in an example of Primark's pornified idea of a T-shirt. She says: "In the Blue Inc shop, they were showing quite a degrading T-shirt of two lesbians. I went in and said to the girl: 'Could you please explain this T-shirt to my four-year-old? Because that's what you're asking me to do by having it in the window.' Poor girl, it actually wasn't her fault. She got the manager, and their line was: 'Well, we don't have to have it in the window.' But they're selling these to men, and expecting it to be in the public space; they're expecting men to be walking billboards for pornography."

As is so often the case, people who would put themselves at the anything-goes end of the spectrum will find their focus sharpen when they look at it from the perspective of childhood. Willows makes the point that you would never have pornographic images in schools; you would never take sponsorship from the Sunday Sport and have naked women along the corridors, or bring in a copy of Nuts to make papier mache – not because of considerations of taste.

You wouldn't show porn to kids because it would be a shame, wouldn't it, for them to start treating each other like pieces of meat before they even knew why. It would be a pity if, before they'd had a sexual awakening, they'd been saturated by a culture in which one gender is a trussed-up, passive sex toy for the other. But this is exactly what we're doing with the rest of our public space. It's not the existence of porn that bothers me, but its colonisation of the mainstream, so that the interests of relatively few people (some grown men; not all men, surely not many women) come before those of everybody else.

Moshi Monsters next to the Sport. Photograph: Childeyesuk via GuardianWitness

Kathy McGuinness, who runs a campaign called Child Eyes, sent in such an awful image that we're not printing it (I can describe it: it's a T-shirt with some CGI rapist cutting off someone's pants. How do I know he's a rapist? You could argue that, as a casual observer, I couldn't say for sure that that wasn't the consensual recreation of a fantasy. But it's plain – and this is what disgusts me about it – that we're meant to think this is an act of violence. This is meant to be where the image gets its frisson, where the CGI dude gets his je ne sais quoi, that he's perpetrating an act of violence against a woman).

She says: "The high street is becoming a no-go area for kids, which is really unfair. Why shouldn't they be able to go into a supermarket, or a newsagent? The people who make the displays aren't thinking about it from a child's point of view. I don't think David Cameron goes to a supermarket with his kids very much. So he can tell us to turn the page"… more shortly on this, one of his more asinine remarks … "but his children don't move in those circles.

Wherever we take our kids, they've just got to look at all these images." And that's one of the things so marked about it, is the way the Sunday Sport or the pointlessly sexually aggressive T-shirts are sold so close to the Moshi Monsters, or the kids' magazines, or the Kinder Eggs (I was struck by the way fairground rides are painted with scantily clad women; when did you last hear of an adult fairground?). The broadcast equivalent would be advertising a sex line on CBeebies. "They surely have a right," McGuiness continues, "not to see all these images. They've got a right to a more positive media environment."

A funfair for adults? Photograph: caroline8899 via GuardianWitness

Lucy Anne Holmes started the No More Page 3 campaign during the Olympics last year. As her colleague, Stephanie Davies-Arai says: "She saw the Sun, maybe she even bought the Sun, because it had Jessica Ennis on the front. She thought out of respect for that, and maybe out of respect for foreign visitors, they hadn't got a Page 3, but it was there on page 13, and it was still the largest image of a woman in the newspaper. So even on the day that a young woman had won a gold medal for her country, someone with her breasts out was still the image of a woman that they considered the most important."

Since then, a number of high-profile organisations have come out in support of No More Page 3, including the National Association of Head Teachers, and the Girl Guides. It was when the Guides added their voice that Cameron said there was no problem with Page 3, parents should just "turn the page". "But the Guides are young women," Davies-Arai points out. "The argument about children is strong, but for me, the group that are most damaged by this are young women, looking at a newspaper and seeing that this is what the mainstream thinks of women. British society says, this is your most important role. People talk about online porn, but without the conditioning that comes from mainstream culture, they wouldn't fall for it."

A Boux Avenue shop window display. Photograph: ChildeyesUk2013 via GuardianWitness

In the end, it's not about which age group is the most vulnerable, it's not really about x-rating or baby-proofing the world so that we ring fence what is and what isn't suitable for under-18s. It often takes explaining it to a four-year-old to wake you up to how toxic it is, but this is still about straight sexism – what is the presentation of a woman in this image? Is she presented as a consumer choice, like an M&S turkey, golden and inviting and usually headless? Is she a person or an object; is she an actor or a prop, does she have any agency in whatever story this picture is telling?

Object and UK Feminista have a high-profile campaign going at the moment, Lose the Lads Mags – as its name suggests, trying to get Tesco and like-sized shops to drop magazines that are branded as lifestyle but are actually mainly about tits. (How is that lifestyle? I mean, if we understand – as I think we must – "lifestyle" to be about ideas for things to buy, how do tits feature, realistically?) Tesco has a policy of not selling "adult" titles – indeed, major retailers hate controversy – but they'll sell images that do the same job because outrage hasn't built up around them. In the post-ironic spirit of the 90s, it became deeply uncool to object to anything. We – actually, I don't know about you, but I got feminist muscle wastage.

Look around now, though – outrage is building.