Timing is crucial. Never more so, it seems, then when it comes to the delicate business of starting a family. Now a new study has confirmed that the pressure on men from having to "perform" on cue can result in impotence and, in a significant number of cases, adultery.

After six months of the stress of so-called "timed intercourse", at least four out of 10 men suffered erectile dysfunction or impotence, and many would try to avoid having sex with their partners at the allotted time, say researchers. Even more disturbingly, the rigours of obligation lead to one in 10 men having extramarital sex, according to new research.

Timed intercourse during the fertile window of a woman's menstrual cycle has been widely adopted and is frequently prescribed by fertility specialists to help couples trying to conceive. Products designed to predict the optimum time to have sex are also commonly used. But having such strictly timed sex can be stressful.

The researchers, whose study appears in the Journal of Andrology, set out to investigate the effects on men, which, they say, has not previously been fully investigated. This weekend they urged doctors to warn couples about the downsides to the technique. "Doctors should acknowledge the potential harmful effects of timed intercourse on men," they said. "Both men and women should also be cautioned about the increased possibilities of erectile dysfunction and extramartial sex.

"Stress incurred by the thought of obligatory coitus, or compulsory sexual behaviour, causes sexual dysfunction in men facing timed intercourse. It imposes a great deal of stress on men, evoking erectile dysfunction and, in some cases, causing these men to seek extramartial sex."

More than 400 men took part in the study, which involved individual examinations by urologists and fertility experts, and a battery of tests. The men, who, with their partners, were using timed intercourse in order to achieve a natural pregnancy, also had their sexual behaviour and any dysfunction monitored over six months. None of them had any previous history of erectile dysfunction.

It is thought that stress generated by having sex at specific times is responsible for the main findings of the study. It is suggested that increased levels of the stress hormone cortisol lowers levels of testosterone, the male hormone that initiates and maintains a man's libido and sexual life. Previous research has linked high cortisol levels to greater risk of erectile dysfunction. "Stress and anxiety are commonly thought to be detrimental to sexual function, and in the present study, as the number of incidents of timed intercourse increased, the number of men experiencing erectile dysfunction also increased," say the researchers. They added that the study also revealed that, as the number of incidents of timed intercourse increased, "more men participated in extramarital sex".

They recommended that couples seeking to conceive naturally should try timed intercourse for three months and then take a break for a few months.

Dr Simon Fishel, managing director of Care Fertility, and one of the world's leading fertility treatment specialists, described the study as important "because it highlights further the social and psychological impact of infertility and its treatment". He said: "Stress affects both partners, and the way they deal with this can affect their relationship and further impact their physiological difficulties."