Is sexual identity now a mere entry in the post-modern menu?

Last week, Simon said the grandmother of his best woman friend had "turned lesbian" - at the age of 70. Which got me pondering about "Sexuality not being fixed" and "Desire having no gender" and "What will the neighbours say?", so I took a Tylenol and forgot about it.

Three days later I'm at a screening of Gazon Maudi, a film about how it takes a single kiss to transform a middle-class wife into a diva dyke, how the lesbian who bestows the lip-lock ends up having a baby with the hausfrau's husband, and how said husband finally makes it with a man. I would have retreated to a darkened room only I was already in one.

Then yesterday Robert rang. After a lifetime of boys, boys, boys, he'd started a relationship with a woman. I hiss something obscene, hang up and hide under the duvet, waiting for the insistent throbbing in my temples to subside.

Cue Diana Ross: I'm still waiting. Well, there's a lot of whatever, thingy, it about. It being ... Give me a second here.

We're not talking about bisexuality (I think). Bisexuality is as quantifiable as heterosexuality or homosexuality. Take dear, sweet, old-fashioned actress Drew Barrymore. In the past year she has married, divorced, gained and shed a new boyfriend, and is now, according to press reports, planning a baby with her lesbian main squeeze, apparently employing the white stuff extracted from REM singer Michael Stipe.

Now, Drew's domestic arrangements do aid and abet whatever, thingy, it. She's helping detonate the nuclear family, to break down monolithic categories. Gazon Maudit swims in the same meltdown: late-20th-century life is something you make up as you go along, as mood and circumstance strike. This can be viewed as liberating, a backlash against two decades of right-wing repression, or as the triumph of market forces. Imagine, sexual orientation - once considered a biological imperative and/or socially conditioned (or ideologically selected, as with "political lesbianism") - may now be a mere entry in the post-modern, multiple-choice menu.

Still, Drew has a label: bisexual. Simon's friend's grandmother wouldn't recognise the branding. She was straight and then she wasn't, just like the married couple in Gazon Maudit, and just like Robert in reverse: never a heterosexual thought, then, you should pardon the expression, bang. Robert says he isn't gay or straight or bisexual (though he may be one or all of the above in the future - who knows?), he's decided he is ... whatever, thingy, it.

No labels then (useful things, labels. You know where you are, what you can do, and - oh, let's be practical - whom you're attempting to shag). As Julie Burchill informed the letters page of the Evening Standard when her liaison with journalist Charlotte Raven leaked: no, she wasn't a lesbian, she just happened to love another woman, a sentiment echoing the formerly Glad to be Gay Tom Robinson's stand when he embraced heterosexuality and fatherhood - he just happened to love a woman. Burchill has since recanted, but she could change her mind again. We're talking mood and circumstance, remember?

And, perhaps, polymorphous perversity and pop culture, too. At one extreme there's Masters and Johnson's research showing that older people (women in their forties and men in their fifties) find it easier to glide from opposite sex relationships to same-sex relationships - out of boredom or in the belief that a change is as good as a rest? At the other, the left-wing think-tank, Demos, which produced a study showing that today's youth takes androgyny for granted: girls who like boys who like boys to be girls who like girls to be boys, as (the aptly named) Blur sing it. And as Pulp and Suede occasionally demonstrate it, despite Brett Anderson, the latter's front man, queering the pitch by stating that he's a bisexual who's never had a homosexual experience. Masters and Johnson imply choice, Anderson negates it; he's flowing with the go. No label, simply a statement for a time when the fluidity associated with femininity is, more and more, a masculine feature, and girls can be boys, simply another costume purloined from the closet and placed in the mainstream's fashionable wardrobe of identity.

And that's not it. Not exactly. I'm under the duvet and all I know is I need certainty and a aspirin. And the phone rings. It's Robert and he says, chill, what's your problem? (Good question.) Look, I'll explain. And I say, I beg actually, shoot, tell me, and Robert says ...