It is an expression as familiar to stag weekend revellers as to international sportsmen: what goes on tour stays on tour. The phrase is said to have originated on an overseas rugby trip circa 1970, but has morphed into a code of honour adopted by just about any group of blokes travelling abroad, whether for sport, business or straightforward hedonism.

There are two rules embedded in the code. The first and most important is that being in foreign climes, thousands of miles from home, frees one from the usual constraints of acceptable behaviour and etiquette, particularly in regard to sexual fidelity. The second is that all exploits must be kept strictly confidential, never to be discussed with anyone outside the group, particularly wives and girlfriends.

The moral relativism implicit in the code has been responsible for a catalogue of misdemeanours, yet no amount of team discipline seems able to thwart the problem. The tabloids have become accustomed to scooping a story or three whenever a party of touring sportsmen leaves these shores, from Ian Botham's various romps to Paul Gascoigne's infamous “dentist's chair”.

Of course, it is important to distinguish between consenting revelry and the allegations that have been levelled against four members of the England rugby union team that recently toured New Zealand. Even the most fevered party animal would concede that things have gone too far when the police are called in. It is also worth remembering that, like many allegations levelled against football players in recent years, the scandal in Auckland may amount to nothing.

All the same, it's quite true that many (though not all) men behave differently when they are away from home in groups. This can often amount to little more than the usual male bonding activities of mild drunkenness and a lot of football banter, followed by retrospectively embarrassing group hugs. Yet, if we are being honest, it often goes a lot farther. I was in the England table tennis team for 15 years and could count on the fingers of one hand the number of players who remained, throughout that period, faithful to their girlfriends or partners.

Me? I was different. That is not to say that I had higher moral standards than the rest of the international ping-pong community, but rather that my debauchery was consistent across time zones. I just couldn't get my head around the idea that shagging a local floosie was acceptable only if the locality was a few thousand miles from the marital home.

“If we're not in the same country, I'm not cheating” was the explanation given to me by one married player (from France, as it happens) after he had conspicuously departed from the last-night party at the world championships in Japan in 1991 with the stunning Lithuanian whom I had tried and failed to pull. His assertion can be dismissed as a load of self-serving hypocrisy (which, of course, it is) but it also has considerable prevalence.

There is probably a bit of Darwinism involved here (there usually is). Scientists have measured, for example, how male fertility varies with distance from one's habitual partner. And guess what? According to the report in the journal Personality and Individual Differences, a man's sperm count doubles when he spends a lot of time on the road - up from 389 million sperm per ejaculation to 712 million. Which, I am sure you will agree, is a lot of extra sperm.

I would suggest, though, that the central plank of the phenomenon is the idea that ethical rules change when you travel. The ramifications of this can be seen as easily in politics and anthropology as on stag weekends in Prague. How often have you heard a politician arguing that the moral imperative for humanitarian intervention is strengthened when a tragedy is happening “on our doorstep”? Chamberlain evoked the same sentiment in a different way when he said that Britain should not get involved in a quarrel “in a faraway country between people of whom we know nothing”.

We seem to believe that our moral responsibilities - to ourselves, our wives and our fellow man - diminish the farther we are from Blighty. Sure, it may be stretching things a little to get Hitler into a piece about lads abroad, but you see what I am driving at? The international sex industry is predicated on this moral obfuscation. Visitors to Eastern European cities may suppose that the prevalence of strip clubs reflects an indigenous curiosity with the female form, but in reality they cater almost exclusively for foreigners. Similarly, if you go to Soho after dark, the seedier establishments are haunted not by the residents of nearby St James's but by foreign businessmen looking for a bit of light relief.

Sex tourism is the sex industry. Without cheap foreign travel, the income of your average prostitute would tumble.

I would be willing to bet (£100 to anyone who can prove me wrong) that if you contacted every company involved in organising stag weekends, you would find that the number of clients booking lap-dancing clubs was proportionate to the distance of those clubs from their local postcode; that the number of adulterous affairs carried on by businessmen correlated with the number of long-haul (as opposed to short-haul) air trips, and that the number of tabloid scoops on the sexual antics of touring British sportsmen rose by half when they travelled outside Europe, and by half again if they crossed at least four time zones.

Does this moral myopia matter? From a political and philosophical perspective, certainly. But from a stag weekend and touring sportsmen's perspective, the usual answer is: “what her indoors doesn't know won't hurt her”. It is a position with many an intellectual hole, but it does explain why groups of men travelling abroad are automatically “under suspicion”, despite their individual protestations of fidelity and drunken phone calls home at 2am.

The promiscuity and rowdiness of the travelling male is not going to go away any time soon, because it has its roots in an idea that is all-too seductive and all-too universal: that when you are far away, the rules no longer apply.

Letting the side down: some sporting highlights:

Paul Gasgoigne in the “dentist's chair incident”, Euro 96

The England team were training in Hong Kong in preparation for the tournament. When they went out to celebrate Gazza's 29th birthday, players took turns to sit in “the dentist's chair”. Teddy Sheringham was pictured with neat tequila being poured down his neck. The tabloids had a field day, running the pictures on their front pages and hitting the team with scathing editorials. Gascoigne got his revenge, though, with a glorious goal against Scotland. Lying on the Wembley turf (right), he allowed the England team to mockingly re-create the episode.

Andrew Flintoff's pedalo

After the opening match against New Zealand in the cricket World Cup last year, Flintoff, who had been dismissed first ball and taken no wickets, visited a nightclub with five other players. He then took a pedalo out in the Caribbean and had to be helped from the sea in the early hours. Duncan Fletcher, the England head coach, compounded his embarrassment by saying that Flintoff had received previous warnings for indiscretions during England's tour of Australia the previous winter.

The All Blacks sex scandal allegations

After an international in New Zealand in 2007, two women apparently offered to sell photographs of an All Black player involved in a threesome. The papers decided not to buy the picture but one of the team was, it is said, clearly visible. The women in question spoke of having sex for up to seven hours, with only short breaks. The story broke after Graham Henry, the head coach, had described his players as “marvellous role models for the country”.

Wallabies 2006 European tour

An Australian TV network named Stirling Mortlock, the rugby union tour captain, as one of a group of players who had apparently been out until the early hours after Australia's 25-18 win over Italy in Rome. Mortlock said later that there was nothing to the incident. “A few guys came home later than we should have and I was one of them,” he explained. The Wallabies had an eight-day turnaround between the match in Rome and their next match, in Dublin.