Katy Guest: If a bunk-up's your bag, Tiger, why tie the knot?

According to Tiger Woods, who has not been true to his own values and the behaviour his family deserves, he tells us, there is one "important and deep principle at stake" in the debacle surrounding his marital "transgressions".

The principle, says his statement, regards the damage caused to his family, not by his behaviour but by other people's intrusion into it. To summarise: "I am deeply sorry that I got caught. I blame it all on everyone else."

Woods is a good-looking, successful sportsman, who seems to have found several partners who were quite satisfied with the odd casual bunk-up. If having occasional sex with multiple women is what makes him happy, I wish him the best of luck with that. But why did he promise not to do it?

Marriage and monogamy are optional. If, religious or not, Woods wants to stand before everyone he cares about in the world and promise to forsake all others, cleaving only unto one partner, until death parts them, he's free to do that. If he wants to sleep around, he can do that instead. But in theory, at least, the one precludes the other. It's not even in the small print; you notice it when you read the vows out loud.

Of course, every man who cheats on his wife believes that his is an exceptional case. But Tiger Woods has company in his delusion. Cricketer and author Ed Smith has defended Woods, saying his fall from grace was caused by the unbearable public pressure for him to be perfect. I'm relieved to be considerably less than perfect, then; I might stand a fighting chance of being honest with a partner.

For some people, though, honesty is not the point. Witness the further trashing of the reputation of Stephen Gately last week following the confirmation that he and his partner, Andy Cowles, had a sexual encounter with a third man before Gately died.

The poisonous comments of the Daily Mail's Jan Moir were supposedly vindicated. Gately's behaviour was called "reckless and aberrational". I mean, seriously, have these columnists never met anybody who's ever fancied a threeway before? I don't see the attraction myself, but I find that group sex between consenting adults bothers me as little as does, say, Mr Moir's aberrational choice of partner. It's horses for courses, isn't it? As long as it's not real horses – obviously.

The only place I regularly encounter people who are mad about threesomes is in the problem pages of some newspapers. I know some couples who have open relationships. Not many, but enough to include gay and straight; married and long-term cohabiting. Sometimes, just like any kind of relationship, they don't work out – but that's about the worst that happens. Stories like Meredith Kercher's are mercifully rare, and I've yet to meet anyone who has caused death by a sex act, but maybe I've just not been around the block as many times as Jan Moir.

As a third way between lifelong monogamy and meaningless shagging around, the route that Gately and Cowles attempted is a tricky one. It's a route, though, that two adults chose together. Tiger Woods's third way through a marriage was spineless, cheap and humiliating all round. But neither story proves anything about the particular types of relationships that these men chose to live in. They just prove a little about particular types of man.

Celebrity shuffle: The Gordon Brown guide to famous faces

You have to feel sorry (again) for Gordon Brown, who surely has enough to stay up-to-date on without having to commit to memory an ever-changing fleet of identikit Hollywood blondes. Brown was in trouble last week when he mistook Reese Witherspoon, who was a keynote speaker at a conference about domestic violence, for Renée Zellweger, who very much wasn't. So for future reference, a brief guide. Witherspoon is the blonde one with the funny chin. Zellweger is the blonde one who looked much more fun as Bridget Jones than she does as Renée Zellweger. Cameron Diaz is the blonde who's occasionally a brunette, just to confuse you. Sophie Dahl is the blonde who didn't look like an anorexic fashion fatality, and now does. Kate Hudson is the one who looks a bit like Goldie Hawn, only younger. Goldie Hawn is the one who looks a bit like Kate Hudson, and more like her every day. And Jacqui Smith is the one who played the Home Secretary between John Reid and Alan Johnson, but with a little more glamour. Next week: how to tell one old Etonian member of the Shadow Cabinet from another, using only a marshmallow-toasting fork and some sherry.

Earth to Dave: I buy my own Aero

Never mind how "unethical and wrong" David Cameron thinks other people's expenses claims are; what on earth was he thinking in May when he developed sudden and overwhelming chocolate urges, and decided to claim them all back on the taxpayer? That month, voters subbed Cameron's office for a 40p Cadbury's Caramel, a 40p Mint Aero, a 45p Galaxy and a 40p bag of Maltesers, as the Tory leader slipped off his shoes, let his hair down and relaxed in front of a weepie movie while his fluffy grey cat/boyfriend substitute curled adoringly round his 15-denier ankles. But what's with this girly taste in chocolate bars? Does Cameron see himself as a Cadbury's bunny caught in the headlights? Is he looking for a lighter way to enjoy Conservatism? Or was he thinking of the Mars slogan: "Think hiding it. Think Galaxy"?

Baby on board – and at the wheel

I sometimes wonder whether some of the anger directed at cyclists might be because people can hear us when we're thinking out loud. Driving a car, you can chunter away happily knowing that your commentary on other road users is baffled by a windscreen and drowned out by Take That. On a bicycle, this commentary becomes public.

I found this out halfway up Herne Hill in south London, when a nice-looking driver edged out of a side road and made me swerve into traffic. "Thanks a lot," I muttered, near her open window. This unleashed such a torrent of filth and fury that SE24 turned blue and the three angelic children in the back of her car committed to memory a further dozen four-letter words. As she screeched away, I noticed the "Baby on Board" sticker. Good job she couldn't hear what I said then.