Article for You Magazine by William Nicholson

Find a moment when you and your husband are alone together and ask him a hypothetical question: 'If you had the chance, and if you were sure no one would ever know, and if you were sure there would be no consequences, would you have sex with another woman?'

'Well,' he'll say. 'That's not a real question. It could never be like that.'

'Yes, but if it was?'

'Why are you asking me this? What am I supposed to say?'

So he dodges and wriggles, and if you persist in the end he'll tell you what he knows you want to hear. 'Of course I wouldn't. I've got you. Why would I want another woman?'

No husband is ever going to tell you the truth. Except me, now. Of course he'd do it. Any man, given a chance of sex with a woman he finds attractive, would take it if he truly believed there would be no consequences. Men desire and enjoy sex in and for itself, without any other emotional connection. How else do you explain the massive male appetite for pornography and prostitutes? How explain star footballers paying for sex when every club they enter is packed with young women who'll gratify them for nothing? Men want sex without strings. Just a bit of fun, and then back to work.

'But they shouldn't!' (Forgive me if I put words into your mouth, it's the writer in me imagining your response.) 'It's wrong, it's selfish, it's hurtful. You may want it, but tough. Zip it up. You've made a commitment. Live up to it. Be faithful to your wife.' And maybe you add under your breath, 'I'm glad I'm not your wife. I'm glad I've got a decent husband.'

This is where things get complicated. I've got history, like most people. I had many relationships through my twenties and thirties. Then I married at last when I was forty. Since then, twenty-two years ago now, I have been faithful. Marriage has proved to be a liberation. I have no desire to return to the freedom I so jealously guarded for so long. I've found a happiness in marriage, in commitment, in fidelity, that is greater than anything I knew before.

And yet I'm the same as any other man. I too, like any husband, would enjoy making love with another woman, if there were no consequences. I don't because there are consequences. Because when I balance up the gains and losses I find I stand to lose too much. Because I no longer get the offers. And because I'm getting older.

There are two more aids to fidelity which have made a big difference to me, but which may not apply to all men. Both are the fruits of marrying late. I've learned from my own experience that sex with someone you don't know, which looks so thrilling from the outside, is simply not as good as sex with someone you know well. And because I was older when I married, I had learned quite a lot about myself; about my desires, my fears, my self-doubts. I told them all to my wife, as if to say, 'Be warned. Are you still sure you want to make a life with me now?' She heard me without surprise, and responded by telling me about herself. For the first time it dawned on me that someone who loves you actually wants to please you, and needs only to be told how. It sounds easy. But I hadn't managed this in twenty years of moving from lover to lover.

Over those same years my central subject as a writer has been love. Even in Gladiator, unlikely as it may seem, my major contribution to the screenplay was Maximus's love for his wife and his longing to be reunited with her after death. Love and its complications continue to obsess me. My latest novel is about a married man who has an affair; which I have never done. And yet he is me. He's me as I would have been had I married younger, and had I not begun my marriage with so much honesty. He loves his wife, his children, his home, but he has never felt able to live out his sexual dreams in his real life. It's the story of what's called a fling – and why it may not be such a big deal after all.

'Aha! Now you're going to start making excuses for unfaithful men! So he has some adolescent fantasies he'd like to act out? Tell him to grow up. Life doesn't give us all our dreams. And if he thinks I'm going to wear stockings and suspenders and let him whip me, he can dream on.'

No, you don't have to dress up and feel stupid. You don't have to be someone you're not. All you have to do is listen to him and find out who he is. If he can talk to you about his sexual fantasies, it's almost as good as living them. What he wants most strongly is not the actual stockings or the actual whip, but to be able to include the sexual excitement he gets from such thoughts in his love-making with you. We men long to know that we can still be loved and desired even when revealing our true sexual preferences.

'But why do they have to be so adolescent? What's wrong with romantic love? What's wrong with grown-up sex?'

All I can say is, it may look infantile to you, but this is strong stuff. See it from his point of view. His waistline has expanded, his hair has receded, and these days he can't always get it up. He meets a woman who wants his body and likes him to talk dirty to her – My God! He's born again! Just once, oh Lord, just once, let me live the dream!

And suppose he does. What has happened? Has he stopped loving you? Is your marriage now over? Of course not. He's not turned into a monster. He's just catching up with himself. He's having a fling.

'But it's not a fling! It's sex! This is the most intimate part of our relationship, and he's doing it with someone else. The very thought of it makes me feel physically sick. And ugly. And old. How can I ever be the same with him again? How can I ever trust him again? And you tell me I'm supposed to forgive him!'

Not exactly. What I'm asking for is understanding. Feel it as he feels it and you won't be quite so hurt.

The plain fact is, it's different for men. Just look at the basic physical differences. For you, for the woman, sex requires you to open yourself up, to make yourself vulnerable. It's something that happens within you, something you receive. And it exposes you to the chance of the longest-term consequence imaginable – a child. For the man it's all the other way round. The act of sex happens outside himself. It's something he throws away. It has no long-term consequences. So he can have his fling and still love you, unlikely though that may seem. He can have sex with another woman and not love her at all. If you can deal with it, this could be your chance to make a far more powerful marriage, based on the truths you hadn't dared tell each other before.

'But how can I know? I feel cheapened by what he's done. I feel I'm worth less. I don't love him as I did. I'm angry with him. I want to punish him. Shouldn't we just cut our losses and part?'

Maybe. Maybe you married young, without really knowing yourselves or each other, and now you've grown up you've found you're not compatible. Maybe this affair is his cowardly way of getting out of a marriage that has made him unhappy for years. Maybe this isn't a fling at all, it's his bid for escape. How are you to know?

I don't think it's hard, if you can bear to face the truth. If you can convince him that you really need to know what he's going through, and that you aren't simply seeking more ammunition with which to punish him and make him feel guilty, he'll tell you.

'But I do want to punish him and make him feel guilty. He's behaved like a selfish bastard. Why shouldn't he be made to pay?'

Because if that's how you deal with him he'll go on lying to appease you. And you need the truth. Is he in or out? If he wants to stay, and you want him to stay, it can be done. Your life together can go on, bent out of its former shape, full of sharp new edges, but intact.

Not very romantic? Actually, I think it is. I think two people learning to love and accept each other, failures and all, is deeply romantic. We're all so needy and insecure, so full of guilt and shame. What we long for is to be known as we really are and still loved. That's a project that takes time and honesty and courage and compassion. That's what I call a marriage.