Numbers, numbers: 17 was the quantity of minutes remaining between the arrival of my late-running Stansted Express on platform 3 and the departure of Ryanair flight 8592 from gate 46. As I walked on board, row 16 had the elusive empty seat at an emergency exit row, which signifies three precious extra inches of legroom. But the lady in front of me grabbed it, as she was perfectly entitled to do – even though she appeared to be only around 5ft 4in and therefore not in a position to maximise the benefit. She also had the regulation one bag. Unfortunately, all the lockers were full, and it could not be stowed under the seat in front of her because she was in an emergency exit row. So the steward cheerfully stuffed it in front of my seat in row 15 – the first time I have had legroom subtracted.

A small inconvenience, which is what you expect on Ryanair in return for cheap flying. And nothing compared with the upset of the poor young woman who boarded the Boeing in tears.

Ryanair romances have flourished across Europe thanks to the Irish carrier offering fares low enough to sustain long-distance relationships. But on this occasion, those whom Ryanair hath joined together were put asunder by the airline.

The lady and her fiancé were planning to marry the following day in south-west France, which is why they were flying from Stansted to Bergerac. But he was stopped at the gate because of a numerical irregularity. So she was obliged to walk down the aisle of the 737, but hopefully not the wedding ceremony, alone.

The problem: online check-in. Most airlines that offer this facility, such as British Airways and easyJet, ask for a minimum of information. Ryanair, in contrast, demands to know the number and expiry date of your passport. If you enter a digit incorrectly, you will be stopped at the gate and prevented from boarding – even if you are due to wed the following day. The ground staff who apply these rules so rigidly are, as they explain, only obeying orders. The procedures are laid down by Ryanair's head office. Which begs the question: why are the rules so strict, and applied so inflexibly?

Sometimes the reason given by staff is "security", though this is the reddest of herrings. Most governments still have no interest in whether the numbers on your boarding pass match those on your passport. (Indeed, for flights within the UK there is no legal requirement to produce ID – the airlines insist upon it only to try to stop people selling unwanted tickets.) The only requirement is that every passenger has been screened by security.

Some Ryanair passengers who have fallen victim to numerical imprecision maintain this is a money-making ruse by the airline. Inexactitude can certainly prove expensive: buying a new flight on the day usually runs to hundreds of pounds. The carrier earns not only this handsome fare, but also pockets all the taxes, fees and charges from your original flight. Yet at the heart (if that is not an unfortunate word on this particular date) of the zero-tolerance policy is Ryanair's uncompromising attitude about its business model. If you, the passenger, do everything right, you will be able to fly reasonably cheaply. Get anything wrong, though, as you jump through the operational hoops, and you risk logistical and financial meltdown.

The boarding pass for my flight to Bergerac read "Gate closes 13:20", which was half an hour before the scheduled departure time. At the time the gate officially closed, I was stuck with plenty of other people on the Stansted "Express" beside a field somewhere in Hertfordshire. But it was still worth a sprint when the train finally arrived, because I have never known a Ryanair flight where the gate has opened half-an-hour ahead of departure, let alone closed. I arrived at the gate nine minutes before departure time, and squeezed through the door just as it closed.

Web check-in saves cash as well as time: £4.75 for every flight. I reckon travellers would grudgingly pay, say, a £50 "fine" at the gate if they get a digit wrong. But in the absence of such a "get-out-of-Stansted" ploy, the gloomy groom was obliged to return to the ticket desk to try to buy a seat on the imminent flight to Limoges, hoping to make it to the Mairie on time for the wedding. The course of true love can expect to be diverted when Ryanair gets involved.