I was at Glastonbury last month, waiting for Diana Ross, when I spotted a grown man queuing up for somewhere called The Milk Stand. He propped his elbow against the bar, rubbed his goatee and went, “Just a carton of milk for me please, mate.” It was 28 degrees outside. And just as the opening chords of “I'm Coming Out” begun, I could see him out the corner of my eye, guzzling milk, little streams of white liquid cascading down his chin. And I thought: 'Sorry? Are you okay?

I'm not the only person who finds the sight of grown-ass adults drinking whole milk unsettling. Type “adult” and “milk” into Twitter and you'll see Tweets like “I do not trust adults who drink milk” and “adults who drink milk straight from a glass disturb me”. Because it is disturbing. It's like seeing an adult man wearing a t-shirt and no pants like Winnie the Pooh, or when people kiss their dogs on the mouth for ages, or when someone over the age of 16 uses the word “mommy” in a non-kink setting.

This isn’t just a “vegan era” thing, either. Adult milk drinking has long been used to evoke creepiness in films. In Stanley Kubrick’s A Clockwork Orange (1971) there's the Korova milk bar, in which Alex and his droogs sup on drug-laced milk in preparation for a night of violence. There's the famous milk scene in Inglourious Basterds (2009), in which Nazi colonel Hans Landa glugs milk before murdering an entire family. In Westworld, the evil android bandits are seen knocking back bottles of milk, even though they don't need sustenance. Milk is used to show that something is wrong. I’m not saying that everyone who drinks milk is a murderous psychopath – but it is unhinged behaviour.

To understand why this might be, let's take a closer look at what milk actually is: It's essentially an emulsion of fat, protein, sugar and water which is produced by the mammary glands of mammals. In humans, milk is formed within the alveoli – grape-like clusters of cells within the breast – which is then excreted out the nipple. Milk, then, is liquid created inside the body, specifically for babies, before they can eat solid food.

With the above in mind, why are you, as an adult, drinking white liquid which was made inside a body? And, worse, why are you drinking the white body liquid of cows specifically? You know what other liquids fit within that category? Cum and discharge.

To check that I’m not being outlandish, I canvas a few other opinions. Many echo my own. “It’s very creepy, especially when they have a milk moustache,” says Hayley Neary, 26. “I had a friend who used to get a bottle of milk every day at school and wrap her mouth around the whole rim and I would have to watch her guzzle milk every day.”

“A big part of it is milk tongue,” says Courtney Meyer. “Like, after you drink milk it just stays there in your mouth. It makes my skin crawl!”

“There’s something sort of sexual and primal about milk,” adds Giorgia Elizabeth, 31. “The idea that adults drink something meant for babies and to help you grow just grosses me out. I associate it with mothers and suckling and boobs and babies. To me, it’s almost like drinking a glass of semen with your lunch.”

Other comments include “adult milk drinkers disgust me” and “even the word ‘milk’ makes me want to be sick” and “it’s not natural”. Who knew there were hundreds, possibly thousands, out there like me, secretly disgusted by a world in which someone might go and buy a little milk cartoon and knock it back like orange juice.

I managed to track down an out-and-proud adult milk drinker. Will Dickson, 29, doesn’t know what all the fuss is about. To him, milk is a perfect drink. “Milk is a rich and naturally occurring source of electrolytes – sodium, potassium, calcium and magnesium – which makes it optimal for hydration,” he explains. “It’s also a great source of whole proteins so works particularly well post-workout. It’s tasty, cheap and available almost everywhere. What’s not to like?”

Sometimes I wonder whether I might grow to appreciate adult milk drinkers. To drink milk straight out of a carton in the hot sun can only be described as chaotic, renegade behaviour. The milk drinker does not care what others think. They only care about their needs. And that is surely commendable.

But then I think about the weird tangy smell of milk when it spills and I want to be sick all over again.