Drinking, smoking, being overweight and taking drugs have little effect of a man's quality of sperm, according to new research.
But tight Y-fronts should be avoided.
Aspiring fathers have previously been warned that cigarettes, recreational drugs, and excessive boozing could reduce their sperm count.
But experts at Manchester University now claim lifestyle factors actually make little difference to male fertility.
Dr Andrew Povey, from the University's school of community based medicine, said: "Despite lifestyle choices being imporant for other aspects of health, our results suggest that many choices probably have little influence on how many swimming sperm men produce.
"The proportion of men who had low numbers was similar whether they had never been a smoke or a smoker who was currently smoking more than 20 cigarettes a day."
The team recruited more than 2,000 men from fertility clinics around Britain who filled out a detailed questionnaire about their diet and health.
Their sperm counts were then analysed and compared to the survey.
Co-author Dr Allan Pacey, senior lecturer in andrology at the University of Sheffield, said men should still take care of themselves, without feeling the need to ''become monks''.
'''In spite of our results, it's important that men continue to follow sensible health advice and watch their weight, stop smoking and drink alcohol within sensible limits.
"But there is no need for them to become monks just because they want to be a dad.
"Although if they are a fan of tight Y-fronts, then switching underpants to something a bit looser for a few months might be a good idea.''
The results, reported in the journal Human Reproduction, showed that men with poor quality sperm were 2.5 times more likely to have had testicular surgery, and twice as likely to be of Black ethnicity.
They were also 1.3 times more likely to do manual work, not wear loose boxer shorts, or not to have had a previous conception.
But men's use of tobacco, alcohol and recreational drugs made little difference, as did their weight as measured by body mass index (BMI).