The pressure to perform when a woman is at her most fertile drives men away, a new study suggests.
The pressure of rigorously timed love making sessions spurred one in ten men surveyed to have an affair, and a quarter claimed it made them impotent.
More than 400 men took part in the study, which found the pressure to conceive a baby caused men acute stress.
Fertility experts tell couples to time sex with the window when a woman is ovulating. There are even devices designed to help couples work out - down to the minute - when a woman is at her most fertile.
But as the number of timed sex sessions increased so did the men's level of stress, according to the research carried out in South Korea. None of the men in the study had ever had sexual problems.
The finding supports several previous studies showing that men who are under stress produce less testosterone, which has an effect on their libido.
The authors suggested couples should be made aware of these risks and attempt timed sessions for no longer than three months at a time, with breaks for a few months in between.
"Timed intercourse seems to impose a substantial degree of stress on male partners, inducing erectile dysfunction and, in some cases, causing them to seek extramarital sex," they wrote in the Journal Of Andrology.
"It is clear that the greater instances of timed intercourse trials, the more incidences of erectile dysfunction and extramarital sex and the greater the desire to avoid sex with the intended partner."
The authors said that having to sleep with their partner at a specific time "becomes a burden and is carried out as a job to be done, which imposes further stress". They believe higher levels of cortisol, the stress hormone, being produced by the body was to blame for lower testosterone.
- DAILY MAIL