Sharp intake of breath here, because everyone immediately gets suspicious, don't they? He must be in love with her, or she must be in love with him, and if they're not already having an affair they will be soon. We're deeply cynical about the idea that men and women can be just good friends because we assume, like Billy Crystal's character in When Harry Met Sally, that "the sex thing always gets in the way". Yet my best friend is a woman and our relationship of the past 12 years is living proof that Plato's ideal, of "a spiritual affection between a man and a woman, into which sexual desire does not enter" (Collins dictionary), does exist.
I met Jackie at sixth-form college in Birmingham. It was my first experience of a co-ed school and when I arrived I couldn't believe my eyes - there were girls everywhere. I kept myself busy pursuing - and being rejected by - a series of the most shallow, superficial but ludicrously good-looking girls. Qualities like quirkiness, integrity and humour didn't rank high in what I was looking for at that age and so I passed over Jackie, who I described to one friend as "the one who was always reading the NME in the common room".
We became real friends when we ended up at the same university. We bonded in a blokey, trainspottery sort of way over our mutual obsession with music, talking music trivia, going to gigs and buying records together. People assumed all the time that we were a couple and when I told them no, they asked why not. I'd say things like we don't fancy each other, or we're too similar or it would spoil the friendship. Deep down, however, I think the simple truth was it just didn't feel right.
When I started writing this article I called Jackie, who now lives in Manchester and asked her about our relationship. "Have we ever talked about being more than friends?" I said. She reminded me of an occasion at university when, at three o'clock in the morning, I once asked her what she'd say if I asked her out. "No," she replied, and that was that. I remember the occasion quite clearly and recall that even at the time I thought it was a bad idea. The truth is that I've had female friends with whom there has been sexual tension and I've used friendship as a way of worming my way into potential girlfriend's affections, but Jackie never fitted into either category. Over the years we've shared a bed together more times than I care to mention. She's seen me in my underwear. I've seen her without make up. She's seen me at my best and she's seen me at my worst and nothing has ever happened. We're friends, not frustrated lovers.
There's a huge difference between my relationship with Jackie and my relationship with my close male friends. I can talk about deep stuff with men but for the most part we use each other to escape from reality - we talk about films, music, work, anything so long as it is removed from the personal.
My conversations with Jackie, on the other hand, are the perfect blend of the inane and substantial. We can talk about Frank Capra films and play our current favourite records down the phone and yet we always find time to over-analyse the important things like love and life. In the early days we learned a lot about each other's worlds. I taught her why men don't call when they say they will and she taught me why the women in my life always insisted on holding a conversation in the middle of Star Trek. Jackie used to say that she was training me to be the perfect boyfriend and she thought I understood women better than any man she knew. Somewhere out there we agreed were a lucky couple of people who were going to reap the benefits of our enlightened relationship. Yet for all the insight, when it came to sexual relationships we reverted back to type. Another indicator that the reason why we could get on so well was precisely because we didn't fancy each other.
When it comes to Jackie's boyfriends I admit I am critical. In an ideal world I'd prefer either to hand pick them for her or have some sort of right of veto. I know what men can be like and I'm quite distrusting initially. Sometimes Jackie will tell me about her current infatuation and I'll reel off a massive list of why they are totally unsuitable. She'll ignore me, of course, but I have to do it. As for me, Jackie would fear for me rather than the woman in question. She would worry that I was heading towards a broken heart (which I usually was).
I remember the occasion when it was really brought home to me just how much I cared about her. It was 1993; I was living in London and she was in Manchester. She called me up in floods of tears. Jackie isn't someone who cries easily and I was shocked. She told me she'd just split up with her boyfriend and was devastated by it. What I remember most clearly wasn't how upset she was but how upset her tears made me. In a fatherly sort of way I wanted to do whatever would make everything all right for her, whether that be stringing up her ex, getting the next train up to Manchester with a crate of Lambrusco or finding her the perfect man.
There came a point when all our friends were in long-term relationships and moving in together or getting married and we thought, after a number of failed relationships, that we might be single forever. But the idea never seemed to bother us and I suspect that was mainly because we had each other. The image of us moving in together as pensioners, still bickering, still talking about music, still friends, was just so cosy. I felt strongly that as long as we had each other we'd never be alone.
Then in 1995 I met someone. Jackie could tell from the way I spoke about her that she was the one. When I told her two months later that Claire and I were getting married, she wasn't surprised. And although she was happy for me, I think she was more aware than I was that our relationship would change from here on. And it did. Settled in my perfect relationship, I had no angst to share. We talked less about me and more about her and for a while it felt that we didn't even do that.
But in other ways my relationship with Jackie has deepened; I like to think the turning point was the wedding day. I know that this could have been construed as a demotion for her but I don't think either of us saw it that way. We both knew a day like this would come as it does for all friends. But I think on our wedding day our relationship changed - from friendship to family. It's a testament to our friendship that when I had to choose my best man, even my closest male friend told me that it had to be Jackie. We won't ever grow apart. It's just not that sort of relationship.
Jackie and Claire get on incredibly well. They share a similar outlook on life, laugh at the same things and deal with social situations in a similar way. I think it was important to both of them that they weren't just acquaintances. Indeed, when Jackie comes round she and Claire will delight in taking each other's side against me.
It wasn't until I married that I fully understood the difference between the love I had for Jackie and the sensation of being in love with my wife - the difference between, as the ancient Greeks put it, filial and eros love. Even though I'm sure we'd all place being each other's best friend as an important criterion in a perfect partnership, without the buzz, that all-important desire to see the other person without their clothes on, it just isn't the same. And the truth is that, although we share many tastes, the main thing Jackie and I have in common is a desire never to see each other naked.
`My Legendary Girlfriend' by Mike Gayle is published in paperback on 11 February, pounds 6.99.