Anyway, the usual boring straitlaced prudes on Twitter got on her case about it. And the poor woman became that day’s Twitter sacrificial lamb, or foal. “Yup, that bestiality tweet was a mistake,” she tweeted later on, sadly, having deleted the original. “I’ll never talk about it again.”

It was interesting, because at one point she made a pretty unanswerable point: unless you’re a vegetarian, you haven’t really got a leg to stand on here. “The fact is,” she said, “you’re being inconsistent if you’re fine with killing and eating non-human animals while opposing sex with non-human animals. They didn’t consent to being killed and eaten yet you’re perfectly fine with it.”

Jonathan Haidt, in his book The Righteous Mind, talks about a phenomenon called “moral dumbfounding”. That is: when something is disgusting, and you want to say that it’s immoral, but you can’t think of a reason why it’s immoral. So you end up simply saying it just is. One example he gives is of a man going to the shops and buying an oven-ready chicken. That evening, before cooking it, he has sex with it. Then he cooks it and eats it. Is that immoral? Apologies if you’re having chicken for dinner, by the way.

Most of us want to say yes, it is immoral. But most of us – especially those of us in the West, and on the left-liberal side of politics – think of morality in terms of whether someone is harmed, oppressed or cheated. And when you’re making love to a dead chicken and then eating it – who is harmed? The chicken? You feel that as far as the chicken is concerned, the worst has already happened. The man eats the chicken himself, so he’s not inflicting violated chicken on anyone – and it’s properly cooked, so no risk of diseases! Who is the victim here?

Disgust is a moral emotion. (I can feel my wife’s moral disapproval when I put mayonnaise on pizza.) But we have, in the West, somewhat separated our sense of morality from our sense of disgust. It is not enough to say “that is wrong because it is disgusting”: you have to say “it is wrong because of [these reasons], and the fact that I am also disgusted by it is an entirely separate and coincidental thing.” 

In the case of the woman fantasising about the horse, the responders were saying things like “It’s wrong because of a lack of consent” or “we evolved to eat other animals”. They were trying to find reasons other than that is disgusting. But those reasons don’t stand up: she had already stipulated that the horse was willing – the horse had consented. That’s right there in the premise of the thought experiment. It’s a fantasy: if horses can’t consent, then she won’t shag one. (Philosophers need to live in the Least Convenient Possible World!)

And the “we evolved to eat them” argument is a straightforward case of the naturalistic fallacy. You can’t get an ought from an is – just because something is natural doesn’t make it right. However you look at it, one suspects that the horse would rather be allowed to have sex with a woman if it chose to, than be chopped up and put in Tesco beefburgers. 

The trouble is that following our moral reasoning all the way through is often extremely uncomfortable. In the case of a woman tweeting fantasies about sex with a horse, it’s merely funny; but in other cases, it leads us into genuinely difficult areas.

For instance, Richard Dawkins recently got in trouble for saying on a radio programme that aborting foetuses at risk of various disabilities, including Down’s syndrome, was morally acceptable: he said that it might plausibly “increase the amount of happiness in the world”.

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I should specify that he was rowing back an earlier comment, from 2014, that it was immoral not to terminate those pregnancies; but again, it hits some deeply uncomfortable note, and we want to say this is immoral. We picture a real child with learning disabilities; we hear from parents of children with those disabilities. We are outraged and disgusted.

But if we are pro-choice, as I am, then Dawkins’ position is implicit. Abortion is already legal in cases of likely foetal abnormality, and 90% of prenatal diagnoses of Down’s lead to termination. It is the woman’s choice, of course: but their choice is presumably made on the basis that either they expect the child would not lead a fulfilling life, or that they expect the cost to their own wellbeing and happiness would be too great. And we (those of us who are pro-choice, anyway) already agree with that. Dawkins merely said the quiet part out loud.

Our visceral reactions drive our moral decisions far more than we might want to admit: for Haidt, the reasoning part of us is usually just coming up with socially acceptable justifications for our emotional gut reactions, a presidential press secretary rather than a policymaker. (As the Matt Hancock scandal was breaking, a friend got in touch to say that he thought a lot of the reason why people were so appalled was because they were sexually disgusted at the idea of being kissed by Matt Hancock, and the rest of it flowed from there.)

According to Haidt, many non-Western or non-liberal people find it easier to say why things that disgust them are immoral: they have a wider sense of what is moral than just “does it harm people?” or “is it fair?” They might appeal to traditions (“not shagging horses was good enough for my father, and his father before him!”) or the disapproval of the community, or simply say this is not what is done. 

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But Western liberals – or more accurately, members of the group known as Western, Educated, Industrialised, Rich, Democratic, WEIRD – need to rationalise out their disgust. Leon Kass, the philosopher, argued in 1997 that we have lost something as a result: that disgust should guide our moral judgments. People getting disgusted at the cloning of Dolly the Sheep or other technological efforts to play god, he says, are “warning us not to transgress what is unspeakably profound … Shallow are the souls that have forgotten how to shudder.” 

And remember, Haidt says that WEIRD liberals tend to think of morality in terms of whether someone is harmed, and in terms of fairness, while non-WEIRD people and non-liberals are more likely to care about other concerns — authority, sanctity and loyalty. And he thinks that liberals should be willing to adopt those broader moral “tastes”: that if, for instance, liberal politicians want to win more votes, they will need to be able to talk to voters in those terms. For instance, they might want to appeal to the moral tastes of authority and loyalty by talking about patriotism; or they might want to appeal to a moral taste of sanctity by imposing strict penalties on sex offenders.

Haidt may be right on the practical aspect, but Kass is wrong on the moral front. We are sometimes disgusted by things that are immoral – but often we are not. People used to be disgusted by same-sex relationships or interracial marriage; most of us are now not. The disgust response was, in some sense, wrong. The work of moral philosophy is to work out, as dispassionately as possible, what is wrong and what is right: it is a difficult task, but not a completely fruitless one, in that most of us would agree that we have made moral progress in some sense over the last few millennia. 

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Emotions like disgust can call our attention to things — we feel disgusted when someone laughs at a homeless person – but they are not reliable guides to what is and isn’t moral. It is worth understanding what drives moral decisions – to understand that some people are disgusted or outraged by things that we don’t, intellectually, think are immoral – but that doesn’t make those moral decisions correct. I can have mayonnaise on my god-damn pizza. You can shag that oven-ready chicken, if you want, although I wouldn’t tell the neighbours. 

And the fact that we are usually not disgusted by the systematic breeding and killing of animals for our food, and yet we are disgusted by the idea of a woman having sex with a consenting horse, is not necessarily a useful way of finding out which is actually more immoral. 

In fact, I will go on record here and say that, yes, morally, it is worse to have animals tortured and eaten than it is to allow them to have sex with you. I’m a hypocrite on this — I do the former most weeks, and the latter only at birthdays and Christmases — but still, I think it is true.