Britain is losing its libido. That is the striking conclusion of an Observer survey – published in the new Sex Uncovered supplement – which reveals that the average British adult has sex only four times a month, less than once a week. Our previous survey, in 2008, recorded a figure of seven times a month.

For good measure, our investigation of what goes on in Britain's bedrooms indicates that a third of the nation does not have sex at all in a typical month, a rise in abstinence of eight percentage points since 2008. Only 1% say they have sex more than 30 times a month, an average of at least once a day.

After six years of recession, and four years of a Tory-led coalition, it would seem that when it comes to sex we have never had it so infrequently.

It is also clear that men suffered the greater blow to their sexual self-esteem as the recession took its grip. In 2008, more than half of the sexually active men who took part in our survey rated themselves as above average in terms of their prowess as lovers. This year, that figure had shrunk to a less than a third.

And there is further bad news for men. Fewer are happy with the size of their penis. An admittedly sizeable proportion – 79% – are still content with their organ's dimensions, against 86% six years ago. Intriguingly, 90% of those from higher social grades report satisfaction with their size, while Scottish men (88%) are also more likely to be happy with their penis.

As to the question of what people are doing instead of having sex, the survey shows that 53% of women (compared with 36% of men) read erotica, in particular novels such as Fifty Shades of Grey. By contrast, 76% of men (compared with 36% of women) watch pornography online. Other intriguing findings in the survey include the revelation that almost one in five Britons (19%) lost their virginity when they were under 16, the legal age of consent. However, the average age at which a person loses their virginity is 18, the survey found, while for those living in London and the south-east that age rises to 19. As to sex education, 19% said they believed children were given too much information at school, while 43% said too little was given. The remaining 40% said they thought pupils got just about the right amount of information.

In addition, 63% of those in the survey said they were satisfied with their sex lives, a decrease from 2008, when 76% said they were happy. It was also found that one in seven (14%) have had casual sex with someone they met on the internet, a behaviour that is most common among those aged between 25 and 34 (28%) and living in London (20%). The survey also found that, on average, people have had 10 sexual partners, a slight increase from 2008.

The survey also revealed a fairly liberal attitude to homosexuality among respondents. Six years ago, 55% of Britons supported the right of same-sex couples to marry. That proportion has now increased to 63%. And there has been an even larger increase in the proportion of those supporting the right of same-sex couples to adopt children. A minority (44%) backed the concept in 2008. Today that figure has increased to 61%

A total of 22% of male respondents – more than a fifth – said they had visited a prostitute. This represents another rise – by four percentage points – since 2008.

By contrast, the figure of 11% for those who have had a sexually transmitted disease represents a drop from 12% in 2008. This figure is mirrored by a continued decline in those reporting worries about contracting sexually transmitted diseases. In 2002, when the Observer published its first sex survey, 52% said they were either very or fairly worried about STDs. This figure fell sharply to 35% in 2008 and has fallen again, to 28% in 2014.

"This decline in concern could be attributed to younger generations growing up in the period after fears of an HIV epidemic abated," the survey notes.

The Observer's sex survey was carried out by Opinium Research between 8 and 14 July, using an online methodology that ensured anonymity.