Julius Caesar, Sigmund Freud and James Dean thought so.
Bisexual persons reckon yep.
And griffin vultures, bottle-nose dolphins, AND LIONS also think bisexuality's for real.
But not all accept the validity of so-called 'fence-sitting'. In fact, some are infuriated by the idea.
Like Julie Bindel, the "journalist, broadcaster, radical feminist and lesbian" who last week dismissed bisexuality altogether and caused a storm of outrage in the process.
In her Huffington Post article Where's the Politics in Sex?, Bindel claims bisexuality is no more than the latest "fashionable trend" (for lesbians), and "temporary lesbianism" (for hetero women).
Challenging "the concept of swinging both ways", she says:
"Lesbians having heterosexual sex are seen as transgressive, when in fact they are simply reverting to a traditional way of being a woman. For a straight woman, having a girlfriend on the side is almost like having the latest Prada handbag."
Bindel cites a study that found "some bisexual women actually doubt whether bisexual women exist at all", and argues, "If bisexual women had an ounce of sexual politics, they would stop sleeping with men".
In other words, stop letting the team down. Just as she presumably did not, when she "made the positive choice to be lesbian". (See: political lesbianism.)
As many have already pointed out, there are glaring holes in Bindel's claims. For one, she assumes that because SHE was able to 'choose' her sexual orientation, everyone else can too. Also, if she IS making a "choice" not to be with men, that infers she is attracted to men - and therefore bi or pansexual herself.
What's more, by dismissing the sexual choices of an entire group of women as wrong, Bindel is no different to the kind of bigot she undoubtedly abhors. The sort who might say something ignorant, like, "You're not a lesbian. You just haven't met the right man."
(Or, as one HuffPo reader describes her diatribe, "This is like the end of Orwell's Animal Farm: 'And they looked at the feminists and the fundamentalists, and they could not tell them apart'".)
Of course, many others before Bindel have questioned bisexuality. Including a widely publicised study in 1995 that reported: "with respect to sexual arousal and attraction, it remains to be shown that male bisexuality exists."
And US dating site OK Cupid, that found only 23 per cent of its self-identified bisexual members actually contacted both genders:
"This suggests bisexuality is often either a hedge for gay people or a label adopted by straights to appear more sexually adventurous to their (straight) matches."
I don't disagree with that conclusion. It's no revelation 'bi' feels safer for some than 'gay'. And thanks to the rise of 'boner dykes' - girls who pretend to like other girls in order to impress men (and I use the word 'girls', because GROW UP) - some women do adopt the bi label to present as sexually adventurous.
But what about actual bisexual people? Who ARE these mythical creatures?
Here's the deal: they need a woman AND a man to be happy. They are sex on the brain sex on the brain sex on the brain, because OMG TWICE THE NUMBER OF LOVERS. And it's just a passing phase, so they can't maintain solid, lasting relationships.
Except, not really! As our very own Anna Paquin could tell you, bisexuality is real, but those things are (common) misconceptions.
What IS true is that society can't seem to make its ill-informed mind up. One day there's no such thing as a "proper" bisexual - the next "we're ALL bisexual", sliding this way and that on the old Kinsey Scale.
Overall, utter confusion.
* Bisexuals are happy with a man OR a woman; they don't need both at once.
* Just because you've had a crush on the opposite gender, doesn't mean you're (necessarily) bisexual.
* Bisexuality is not born of greed.
* Bisexuals aren't always equally attracted to men and women.
* Bisexuals often struggle to come out, just like gay people.
* Bisexuality is not a choice.
* Bisexuality is not always a phase.
* Bisexuals are as faithful (and adulterous) as straight people.
They're just a few facts, but they're little known. And ironically, if Julie Bindel hadn't had her giant rant and drawn attention to the matter, many of us would still be in the dark.
Or some kind of biphobic fog.
Incidentally, if your name happens to be Sarah, you're bisexual (or you believe in bisexuality), and Bindel's views don't sit well, have I got the Very Niche Facebook Page for you. 400 Women Called Sarah aims to counter Bindel's inflammatory views by finding 400 bisexual or bi-supportive women called (surprise!) Sarah.
Inspired by 'Project Steve' which defied a group of scientists who doubted evolution by producing a far longer list of scientists called Steve who supported it - there are more than 500 Sarahs thus far.
Says one of them: "I'm a Sarah, and I'm currently in the process of writing an article which claims Julie Bindel doesn't exist."
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What are your views on bisexuality? Is Julie Bindel's story offensive to the bisexual community? Are you personally offended? Let me know below.