By Tom de Castella, BBC News Magazine
After another sex scandal involving a senior member of the Catholic Church, questions are again being asked about celibacy. Is it realistic for someone to permanently go without sex?
Celibacy does not mean abstinence.
To the purists, celibacy - derived from the Latin for unmarried - means a permanent state of being without sex.
Abstinence can be temporary. And it's possible to be abstinent in a relationship. "True" celibacy means a life without both sex and a spouse or partner. Of course, there are many who give it a looser definition - merely indicating some sort of commitment to be without sex.
The subject is back in the headlines after Cardinal Keith O'Brien admitted that his "sexual conduct" had fallen below the standards expected of him amid allegations of "inappropriate behaviour".
As a Catholic priest he was expected to abstain from all sexual activity and devote himself to God and the Church's followers. Buddhist monks have similar expectation. In both religions, masturbation is regarded as a breach of celibacy.
For non-religious people the institution can be hard to comprehend.
Catholic priests are all men and while there are celibate women - typically nuns - much of the debate tends to focus on male celibacy.
Taken in its strictest definition, there is a question mark over whether celibacy is possible.
Men are driven by testosterone to want sex, says John Wass, Professor of Endocrinology at Oxford University. Women are driven to a lesser degree by a mixture of testosterone and oestrogen, he explains. "I'd regard celibacy as a totally abnormal state."
Around 80-90% of men masturbate and it's likely that priests do too, he says.
There is data to suggest that men who ejaculate more are less prone to prostate cancer, he says. "You could argue that it's not so healthy to be celibate."
Many people simply cannot imagine, purely on a physical basis, going their whole life without sex of any kind.
Jimmy O'Brien, who left the priesthood to start a family remembers how difficult it could be for young men. "You have to fight the urges. For a lot of people it can be a daily battle, others are not so affected."
The power of the mind through exercise like meditation can banish physical cravings, argues Vishvapani, a Buddhist. "There's no doubt in my mind that some people are able to practice it quite happily. It may sometimes be a bit of a struggle. But the idea that biologically you can't - that's false."
Father Stephen Wang, dean of studies at Allen Hall Seminary, says it is a sacrifice that many priests manage. "It's possible when people have an inner maturity and the faith and support structures are in place." For him it is no different to the challenge of a husband trying to be faithful to his wife, he argues.
There is no celibacy get-out in the form of masturbation, says Wang. "For every Christian, masturbation, sex before marriage and sex outside marriage are wrong and something you shouldn't be doing.
"Masturbation is forbidden for every Catholic. The reason is that it makes us more selfish, more introverted and less able to open your heart in love to other people."
Of course, there are many millions of Christians who would disagree with Wang's position.
It's not just biology, sexual chemistry makes celibacy a difficult lifestyle, says Jimmy O'Brien. Women sometimes saw priests as "forbidden fruit" and a bit of a "challenge", he remembers. But what he found hardest was the loneliness.
"We're only human and there's an element of loneliness. A lot of us need that significant other in life."
Western society dwells on the huge importance of the search for a romantic life partner. Giving up the idea is a huge sacrifice.
"All the intimacy of sharing life with someone who is fundamentally on your side - all that you're denied," says Vishvapani. He is married because he too wanted that significant other in his life.
Modern life is sexualised and individualistic, he says. People in past centuries were either married, in which case they could have sex, or celibate if they were not. Now the options are more varied.
"The idea of being single and sexually active just wasn't possible for people in traditional society. People were more willing to accept a role, such as for priests to be celibate." As a result, numbers willing to make a vow of celibacy are declining in the West.
Plenty of Catholics, including Cardinal O'Brien, have called for a rethink on celibacy.
But for Vishvapani the problem is not celibacy but the sense that it must be enforced for life. "The problem comes when people can't sustain it but don't have any way of being sexually active that isn't unethical."
There's also the question of why certain people choose a celibate life. In a less-than-tolerant society many gay people might choose the priesthood because it would be somewhere for them to hide from sex.
Whether celibacy is physically possible or not, the problem comes when it is institutionalised, some argue.
Forcing priests to suppress their urges or hide their sexual behaviour has warped people, believes Elizabeth Abbott, author of A History of Celibacy: "For thousands of years it's failed. It brings out horrible things."
Jimmy O'Brien says the next pope must look at the issue of celibacy. He has been married for 23 years and believes he made the right choice.
"Having experienced the contentment of family life I'd say I have more to offer the Church now than I did back then."
But Wang argues that people misunderstand celibacy. It ensures a unique relationship with God and one's parishioners, he says.
"It's not about repression. It's about learning to live in a certain way."
It's not just priests who are called by the church to be celibate, it's everyone outside wedlock, he argues. He rejects the link, commonly made in the media, between celibacy and scandal.
"It's not true to say that celibacy leads to sexual dysfunction or abuse. Unfortunately sexual scandals are occurring across society in various organisations, and feature married men not just celibate people."
The central issue is not about belief, says Dr Sandra Bell, a lecturer in anthropology at Durham University and author of Celibacy, Culture and Society.
"It's not an intrinsic belief in the Catholic Church, it's a law. When Anglicans want to convert to Catholicism they can keep their wives, which shows it isn't really a religious belief for priests to be celibate."
Gandhi and celibacy