How does someone become heterosexual? Did something go awry in their childhood? Is there a straight genetic code? Could it be learnt behaviour, aped from famous heterosexuals? Or is it hormones? Whatever, it is probably the fault of Jeremy Clarkson.

These questions – parodic, idiotic – continue to be asked about gay people, argued on a loop even a drunk would deem repetitious. They form an inescapable interrogation, with biologists, neuroscientists, quack therapists, wack columnists and a flotilla of bigots holding the lamp.

Except now someone else has stormed into the room, and she is Clarkson’s worst nightmare: a fighty, fuck-you, radical feminist lesbian, who writes for the left-wing papers.

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Julie Bindel, 52, journalist, campaigner and searing pain in the patriarchy’s arse, has resurrected the controversy about the causes of homosexuality in the most contentious chapter of her new book, Straight Expectations.

It is contentious – a hand grenade, really – because in it she asserts, like an Old Testament maven from Tennessee, that being gay is a “choice”, it is not innate and no one is, in Lady Gaga’s words, “Born This Way”. Middle-fingering the prevailing liberal orthodoxy, she claims to have “made a conscious and happy choice to be a lesbian” more than 30 years ago because she had “better taste than other young women”. Pow!

Most gay people disagree with her, wildly, including me, believing that homosexuality – like heterosexuality or left-handedness – chooses; it is not chosen. Aside from the very obvious reason – why opt for oppression? – sexual arousal, governed by the unconscious, non-cognitive, sympathetic and parasympathetic nervous systems, is nothing to do with the part of the brain responsible for decision-making – the lateral frontal pole. Thus, there is no à la carte for orientation. I did not order the coq au vin.

And so, as Bindel plumps down on to the distressed couch in her living room in north London, holding a cushion protectively in front of her, the first obvious question arises: when exactly did she choose to be attracted to women?

“I didn’t,” she says, rather confusingly given the statements in her book. “Because I needed to leave home – there was nothing there for me in Darlington – and pursue my feminist possibilities, that meant starting a new life and all that was open to me. I fell in with a crowd [in Leeds] who spoke about lesbianism as part of women’s liberation. I never chose to be attracted to women.”

So why say in the book that being gay is a choice?

“Because I think the opposite of having an innate, biological explanation [for homosexuality] – there’s no evidence for that – has to be some kind of choice, as well as some deep-rooted, embedded responses that developed through different experiences in our childhood.”

This is exactly what therapists who claim they can “cure” gay people say. Bindel insists, however, that because scientists have failed to conjure proof of biological predisposition, homosexuality must be a decision.

“It can’t be that there is a difference in our brains, because it would have been discovered by now,” she says.

Why? Scientists know very little about the brain.

“Because it has obsessed scientists for a long time…”

So has cancer, I add, but Bindel presses on: “I could have stayed where I was in Darlington and gone along the path that was predestined for me [marriage to a man]. I chose to live my life as an out and proud lesbian.”

So could she have stayed and “chosen” to be attracted to men and marry one?

“I don’t know. I’ve got no idea,” she says, shifting awkwardly on the sofa. “I don’t think it’s a choice like which washing powder to buy.”

But when you use an explosive word like “choice” that’s exactly the kind of thing people think of.

“Choice is the opposite of innateness,” she offers.

No. No, it isn’t, I reply angrily, you are simply saying it is. The atmosphere hardens.

“I’m not the only person saying this,” she points out. “I’ve met a huge number of lesbians who say, ‘I don’t believe I was born that way, and I believe any woman can be a lesbian and believe we’re stopped from feeling sexual attraction to the same sex because of external pressure’. For women who live under a compulsory heterosexual model, it can be liberating.”

But that is a different issue. How can sexual arousal, an instinctive response, be cognition?

“Do you think sexual responses are the same as sexual identity?” she asks. “People have all kinds of responses which aren’t linked to their sexual identity.”

OK, then, what evidence is there that gay is a choice?

“I have about as much evidence as you do, which is the testimonies of people who say, ‘I could have gone either way but I love being gay’.”

But that isn’t as much – many more people feel that it is innate.

“There’s a reason for that – we internalise doctrine. We are told constantly we are born that way.”

This is slippery, of course, only believing the testimony of the minority who agree. And to suggest that gay people are merely passive robots, regurgitating Lady Gaga mantras, is immensely patronising. But Bindel asserts that so few people openly agree with her because her position is “extremely unpopular and threatening. You really get flak for saying it”.

Her appraisal of the causes of homosexuality has been “drowned out” by scientists “obsessed” with the question and gay people who use the notion of a gay gene to provoke “sympathy”.

“I get massively irritated with this [adopts whiny voice], ‘please feel sorry for us, don’t be horrible to us, because we can’t help it’ [discourse] as if it’s wonky wiring.”

That is grossly unfair, a total misrepresentation of gay people, I all but shout. Her dog starts barking loudly in the next room. But the born-this-way argument, she rages, indignant, “will backfire – it capitulates to bigots, and lets them set the agenda. We deserve our rights however we came to be gay”. Mimicking the “choice” language of bigots is what will fuel their agenda, I reply.

“I don’t see what comfort it will give bigots for me to say being a lesbian is great,” she says.

They will say that even a prominent gay-rights campaigner agrees that it’s a choice, I counter.

“But I don’t agree with them! They wouldn’t use an argument from me in a million years!”

She goes on to explain that environmental factors, such as “opportunity and luck”, trigger gayness, or allow a person to “further explore a sexual attraction rather than shut it down”.

If this were the case, if oppression stopped people experiencing gay feelings, then surely in places of extreme anti-gay hostility, say, in Uganda or Cameroon, there would be no gay people at all?

“I don’t know,” she replies. “All I know is I’ve never been convinced by a scientific argument, or seen any evidence that is compelling that there is something innate about our sexuality. What I’m suggesting is, there are people who could go one way or the other and happily choose to be lesbian or gay.”

That is a bisexual who is simply making a choice out of the options available. That is not choosing which sex triggers arousal.

“But how does innateness explain bisexuality?” she asks.

Bindel does not accept that anyone could be innately bisexual either. After some increasingly tense exchanges, it becomes clear that what she means by choosing to “be” gay, in fact, is “doing” gay. In other words, whatever the underlying feelings are, making a decision to have a gay relationship, identify as gay, come out and lead a gay life (whatever that is).

Politics trounce feelings. In the context of a deeply oppressive world for women, she understandably deems such a decision a “positive choice”. But does that mean that “choosing” heterosexuality is a negative one? “I’d like more heterosexuals to think about why they’re heterosexual,” she replies.

I begin to wonder whether it’s to get away from Julie Bindel.

“Most don’t know they are – like white people recognising they are white. But I don’t think every woman should be a lesbian.”

Bindel admits that far fewer gay men agree with her than lesbians do, so are there fundamental differences between male and female sexuality?

“No,” she says, “there’s huge amounts of conditioning.”

Do hormones not affect mood, energy and behaviour?

“I’m sure they do,” she says.

In which case, given men have more testosterone, would that not account for differences?

“You can’t see hormones in isolation from other factors such as environment,” she replies.

Several studies have shown that women, regardless of their orientation, respond to erotic images of both sexes – unlike men, who respond according to their orientation.

Does she refute these findings that suggest greater fluidity among women, which might then explain why some feel they “chose” lesbianism?

“It sounds biological essentialist to me. It’s yet again another separation of male and female responses, to show that women are somehow different.”

It seems when science lacks, as with finding an organic cause for homosexuality, Bindel jumps on it, but when science does present evidence, she pooh-poohs it.

For Bindel, gender is entirely a social construct – a common view, but she seems keen to deny any effect biology might have, which feeds back into her denial of any organic explanations for homosexuality. All of which stems from her radical and separatist lesbian feminist background in the late 1970s and 1980s. Bindel wrote in 2009 that she hadn’t had a male friend until 1999, and in 2006: “I hate men... But only the men who perpetrate these crimes [of violence] against my sisters.”

As with many polemicists, however, it is a mistake to assume that the frothing person on the page is the person in the flesh. To meet Bindel socially, as I have several times, is to be bombarded with warmth, like a soft, cuddly toy dropping down in an arcade: pale blue eyes, kind and sparkly, a dusty voice calling you darling and a filthy laugh that could earn dearly on a sex-line should the whole writing thing go up in flames (and, of course, if she didn’t deem sex work a violent byproduct of oppression).

As Britain’s punchiest lesbian feminist commentator, Bindel shines as a Sapphic sapphire, refracting stereotypes about gay women as they’re projected onto her weary, raised-eyebrow self: the militant, the man-hater, or the Greenham Common hippie weaving endless orgasms out of hemp and leg hair, shimmers back as someone, on the surface at least, rather more akin to a stereotypically camp gay man: cocktails, gossip, a jolly nice shirt.

Much of Bindel’s career has involved battling violence against women – she founded the campaigning group Justice for Women with her civil partner, the lawyer Harriet Wistrich, in 1990. And most of Straight Expectations, her first book, is equally worthwhile – a stringent examination of the state of the gay nation: the untold story of anti-gay hostility beneath the confetti of legal equality; the suburbanisation and nauseating commercialisation of gay life. Here, she is on strong, righteous ground. On the choice issue, I fear it is molten.

It would not be the only volcano on which she has sat and been burnt. A decade ago, after writing a hateful diatribe about transgender people (for which she has apologised and today calls “plain wrong”), opining that “men disposing of their genitals... does not make them women” and ridiculing trans women for “fuck-me shoes and birds-nest hair” she was subjected to an endless campaign by some trans activists. Death threats, picketing, petitions, comparisons with Hitler, “being told to die in a fire” and misogynistic abuse all followed.

She won’t be drawn into further discussions about her views here except to say, “I want people, men and women, to feel comfortable in their skin”.

But in other areas, unbridled to the end, Bindel breezily sharpens the dagger. In her book, she lambasts Clare Balding for commentating at the Sochi Olympics with scarcely a protest. Today, she rails at Gareth Thomas, the first rugby player to come out, because: “He spent all of his career being in [the closet] and then all of a sudden he’s given every award because you’re supposed to live up to some macho ideal and by saying you’re gay that’s a brave thing to do? I’m not impressed.”

And she accuses Caitlin Moran (the columnist and author of the best-selling feminist memoir How to be a Woman) in her book of using gay men as “accessories”.

Now, leaning forward, having disposed of the cushion, she describes Moran’s tome as “How to be Caitlin Moran”, adding, “it’s not about feminism, it’s about an extremely strong individual” and, “you don’t find out you’re a feminist by putting a hand down your trousers and feeling if you’ve got a vagina”. With that, her dog finally stops.

After nearly two hours of arguing, Bindel sees me to the front door and sighs loudly, pushing the floppy fringe out of her eyes.

“Bloody hell, you’re relentless,” she says, exasperated. I suspect it’s a compliment.

‘Straight Expectations’, by Julie Bindel (Guardian Books, £12.99) is out now