"Not tonight, honey. I have a headache." That's a standard comic line used in movies and TV sitcoms to portray the dwindling sex life of middle-age couples.

However, while usually it is the wife who says those lines, a growing number of Australian males are using it as well to escape from their husbandly duties to their wives.

A University of Sydney PhD study with 300 male respondents found that one in six Aussie men wants sex only once a month due to tiredness and stress. And 46 per cent pointed to stress and fatigue as the culprits, while a smaller number blamed in on dissatisfaction with their partners, relationship problems, medication or illness.

"It's not always the woman whose libido goes out of the window; sometimes it can be the man," Sexology Australia Director Elaine George said, quoted by The Sydney Morning Herald.

"There is a myth that all Aussie men want sex 24/7 and if they don't there is something wrong with their virility and masculinity. This research suggests that for one in every six couples the woman is likely to have higher desire than men," she added.

On the opposite end, she explained low female sex drive to be caused by fatigue, a newborn baby and oestrogen depletion because of breastfeeding or menopause.

In another study published in the latest edition of the Medical Journal of Australia, chances of erectile dysfunction goes up by 11.3 per cent yearly once males reach the age of 45, according to researchers.

However, they warned that the risk would become higher if they smoke, drink heavily, have a sedentary lifestyle and suffer from chronic ailments such as diabetes and heart disease and depression.

David Smith, associate professor and researcher from the Cancer Council of New South Wales said current heavy smokers have 86 per cent higher chances of suffering from ED problems than other males, while obesity doubles the risk.

However, by the time they reach 75, 80 per cent have moderate or complete ED, he added. The study has more than 101,000 NSW male respondents with no previous diagnosis of prostate cancer and aged 45 and above.

It found 39 per cent had no ED, 25 per cent had a mild case, 19 per cent moderate and 17 per cent were suffering from complete ED.

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