Affairs are no longer the preserve of Bad People. I know this because there's a book out called When Good People Have Affairs, by Mira Kirshenbaum. Just because you find something on the internet don't make it so, but when it arrives in actual print, you have to sit up and listen. It's not so simple that you can just put the paper down, go out, shag someone from the corner shop and remain Good. I think you have to be acting under one of Kirshenbaum's 17 motivations, which are; breaking out into selfhood (my personal favourite); accidental; sexual panic; let's kill this relationship (and see if it comes back to life - her brackets); mid-marriage crisis; trading up (this is when you've "moved on" and your spouse hasn't. It really means "lost weight"); heating up your marriage; I just needed to indulge myself; ejector seat (this is exactly as it says on the tin - it helps to get you out of your marriage, much like techniques such as "doing a runner" and "getting a divorce"); see if; distraction; surrogate therapy; do I still have it?; having an experience I missed out on; revenge; midlife crisis; and, finally, unmet needs.

I think there's a certain amount of crossover between these motives, but I am not here to critique this remarkable addition to human knowledge. I am here to add to it yet further. There are even more Good People out there who have had affairs for none of these reasons - whose motives were even more contemporary. But that doesn't make them Bad.

I was binge-drinking

You are happy in your marriage and love and respect your partner. You have no desire to break out, your needs are mostly met, you do not want an ejector seat, you have nice times both in the week and at weekends. And yet, there you were, really drunk. Man, you were absolutely gone. Earlier on, you'd been trying to chase some teenagers who were having a fight because you thought you could make them see the futility of violence just by smiling and holding your arms out. Who would expect fidelity from a person in this condition?

I was bored at work

Ha, your work isn't even all that boring. But often you would rather chat than do it, and the person you're chatting to largely knows what you're talking about, since you do the same job, ergo understands you better than your partner, ergo seems like a person you might have a special connection with. Once you suspect a special connection, every force in the known universe compels you to check.

I'm going bald

Let's say you started your marriage with hair, and you were a Good Person. And now you have very little hair. You are staring down the barrel of having to shave your remaining hair off and meet mortality full in the face. You are still Good, you haven't got any worse. And yet your standing in the partnership has gone down, since in all likelihood, your wife (or husband) still has hair. Of course you must have an affair.

I'm menopausal

When you are a teenager, they tell you a lot about the menopause in biology, and it sounds like the worst thing that could possibly happen to anybody. And then, when you listen to Woman's Hour as an adult, they tell you constantly that all those bad things (and they are a lot to do with libido, and the actual, you know, nuts and bolts) might not necessarily happen. So, people, we are getting a lot of conflicting information here, and how, in a marriage, especially one of very long standing, are we supposed to check? Impossible. Must find new fancy. Must test apparatus.

I was just being polite

Beyond all other hates, I hate sex generalisation. But I think sometimes men are even more prey to this than women are. I have definitely propositioned a whole load of married men who would rather have gone home. Only joking! People say "just being polite", but there's no "just" about it. Courtesy is the oil that turns the wheels of civilisation. If the individual marriage sometimes has to be sacrificed upon its altar, what would you prefer? Anarchy?

I wanted a baby

Yes, yes, in an ideal world you would get pregnant by the person you're married to, and that way you could have a legitimate child in a secure family unit, and studies show that its educational prospects would be excellent and it probably wouldn't end up in prison. You'd be able to take a DNA test without fear. You wouldn't watch your child hit adolescence and feel a chill down your spine as you realised he or she looked like someone altogether different. In an ideal world, that's what you'd want for yourself and your loved ones. But on the other hand, you've been trying for ages and just before you go crazy with the clomid, what if Richard Dawkins is right? What if a little variety might help?

I'm pregnant

I wouldn't just rework a joke out of Sex and the City if it wasn't 100% true. If you get the second trimester horn and your spouse for whatever reason won't step up, then you are within your rights to go elsewhere. To deny you would be the inhumanity of the high-minded. They probably wouldn't approve of dope smoking among the terminally ill, either. The same applies in the third trimester, of course, though good luck with finding a volunteer.

He was famous

Or she. My fella is allowed to do Kelly Brook, and it's not because I know he'll never meet her, and if he did, she probably wouldn't. It's honest to goodness because she is famous, and it would be a shame not to. It was dark and I was confused Come on! Wouldn't anyone act the same, under these conditions? Are you really going to make this a deal-breaker?

I wanted to try my own gender

We're all under a lot of pressure, these days, to experiment. Surveys repeatedly show that the people who report the greatest pleasure from sex are those most likely to have had a same-sex experience. How are you supposed to know if you're in that top five percentile if you don't even try?

Everyone else was doing it

Isn't "I'm married" quite a lame excuse for not doing it, if the person asking you to is also married? Wouldn't a Good Person have to do a bit better than that?

I find it hard to say no

It's not so much that you don't like to say no, more that you like to say yes. I blame the TSB ad for our whole generation.

I didn't have enough change for the bus

I would balk at this excuse, in our Oyster card age. I would say, "Matey, I do not believe this is Good Person Adultery. This sounds a lot like Bad Person adultery."

Because a new self-help book said it would save my marriage

Anyone who wants to report back, please do. We could do a cohort study.