BELIEVE it or not, not all men want sex on tap. Actually, some hardly want it at all. And apparently that's normal.

It's widely believed that men are always up for it. But a sex researcher reckons that men just can't say 'no'.

Andrew Smiler, author of Challenging Casanova: Beyond the stereotype of promiscuous young male sexuality, says men don't realise they can actually refuse sex.

"One of the things I've learned is that most, or even all, guys don't know how to refuse sex," he said.

"When I've asked specifically about refusing sex, the guys tell me that part of why it's hard to say 'no' is that they're concerned about how she'll respond, including concerns that she'll think he's either some type of wimp or gay," he said.

"Those comments have always come from heterosexual men and almost always about a relationship in the early stages."

Dr Smiler, former president of the Society for the Psychological Study of Men and Masculinity, recently wrote A Guy's Guide on When to Refuse Sex for the Good Men Project about when men should refuse sex with a new partner.

"Our cultural assumptions about male sexuality don't allow for the possibility that a guy would ever choose to say 'no'," he wrote.

"In fact, you may have a hard time imagining that a guy would ever turn down sex unless he was in the middle of a massive heart attack.

"Let me be clear about one thing. Guys are allowed to refuse sex by saying 'not now' or just 'no' and their partners need to respect that decision."

Dr Smiler said his empirical work on sex has been entirely with young heterosexual males.

He said guys are often uninterested in sex if they're tired, sick or too drunk to perform sexually.

"For older guys - typically aged 40 or over - other physical conditions start to come in to play including obesity, heart conditions, pulmonary issues, anything that impacts blood flow and a variety of other disorders," he said.

A recent survey for online pharmacy ukmedix.com found 62 per cent of men turn down sex more frequently than their female partner, with a third admitting they had lost their sex drive, reports the Daily Mail.

They report that another poll reveals one in four men no longer has sexual intercourse at all - and the figure rises to 42 per cent for men over 55 - while a quarter said they had been affected by erectile dysfunction at some point in their lives.

Dr Malcolm Caruthers, founder of the Centre for Men's Health, told the Daily Mail that testosterone deficiency is becoming more common and happening younger.

"It used to be mostly men in their 50s, but it's now men in their 40s, or even 30s. Large studies done in America show that every decade there's a decrease in testosterone levels by as much as 10 per cent. I believe the same is happening in (the UK)."

Even rock star Robbie Williams has struggled to maintain a sizzling sex life.

The singer, who has been open about his low libido, said in a 2011 interview that he injects himself with testosterone twice a week to boost his sex drive.

The 39-year-old said a doctor told him that he had the "testosterone of a 100-year-old".

He's not alone. News.com.au spoke to a couple of ordinary blokes about their sex-lives.

One man said his sex drive was higher than his wife's.

"Although this has changed over the years. Speaking to other blokes I know, I think women's sex drive peaks in their early 30s then drops off in their 40s - partly for biological reasons I guess and partly because of kids/exhaustion," he said.

"I think that because of television shows like Sex and the City and other cultural influences, women don't feel cool about admitting this."

The 43-year-old said he never refused his wife's advances.

"I used to, but again, like most men I think I now have a 'take it when it's available' kind of mentality.

"It's definitely OK (to refuse). In a mature relationship, partners understand that you may just not be in the mood or have stuff on your mind or whatever. It can still feel like a rejection, but not devastatingly, underminingly so."

The father-of-two said women need to know that their formula of "intimacy first, sex second" doesn't always work for men.

"Seems you always want a perfect meal and a kind of perfect emotional state of togetherness or something before you agree to hop in the sack with us," he said.

"For men it's often the other way around. We want the physical closeness and release of sex, and then relish the emotional closeness that follows.

"For men, sex brings us closer to you. We wish women would recognise this and be prepared to treat sex not as the ultimate expression of emotional closeness, but as a physical act which is a great way of helping achieve that closeness."

Another man who chatted to news.com.au said his girlfriend had the higher sex drive, but that he never refused sex.

"Even if you're not in the mood for sex to begin with, it isn't exactly difficult to get in the mood. It's not as though sex is a chore," the 23-year-old said.

"I'd imagine it'd be OK to say no as long as I explained myself properly. Refusing sex without offering an explanation will only make your partner feel inadequate or insecure.

"Women need to know that men aren't always horny. So if your partner isn't keen to have sex at any given time, it doesn't necessarily mean he doesn't find you attractive - it could just mean he's not in the mood."

According to Sydney Men's Health clinic testosterone is responsible for the peak in sexual interest in men around the age of 20 and women in their mid-thirties.

"The ageing process in men and women reduces the available level of testosterone resulting in a natural decline in libido in the older years. However it has been found that an older man's libido may not necessarily be related to his level of testosterone," their website says.

"A common cause of low libido is not related to lack of production of testosterone but rather due to relationship problems, such as when a decision is required for a long-term commitment in a new relationship. Any medical condition as well as excessive alcohol intake may contribute to reduced libido."

Other reasons men aren't in the mood (according to an office poll) include exhaustion, energy levels, overtraining, sickness, stress, wanting a sleep-in and eating too much chocolate mouse for dessert.

Continue the conversation via Twitter @AndrewSmiler | @newscomauHQ

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