Britain has become increasingly open-minded about sex in the past generation but less tolerant of infidelity, research has revealed.

As a nation, British people are now more accepting of sex before marriage, same-sex relationships and one-night stands than they were in the 1980s. But the concepts of monogamy and faithfulness are more deeply entrenched than ever, with just one person in 10 supportive of partners who are unfaithful.

Laura Watt, a sociology researcher at the University of Manchester, analysed data going back to 1990 from the National Survey of Sexual Attitudes and Lifestyles, which was set up to study the spread of sexually transmitted diseases, including HIV. She discovered that the number of respondents aged between 18 and 30 – the age group most likely to reveal changing sexual behaviour – who felt that casual sex was wrong fell dramatically from more than six in 10 to just 37 per cent in a decade.

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Attitudes towards gay partnerships changed even more markedly, with opposition nearly halving in the same period and acceptance of same-sex relationships rising to around 70 per cent. By contrast, nearly nine in 10 people said it was wrong for a married person to have sex with someone other than their spouse, with more people saying it could never be justified in any circumstances.

Around eight in 10 people felt it was wrong for people to have more than one partner if they were living with someone – up 4 per cent during the same period. And there was a growing insistence on monogamy within relationships even when couples were not co-habiting. The number opposing the idea of multiple partners in this type of union rose by 7 per cent.

Ms Watt said the studies and other research she carried out among young people in Preston, showed a revolution in young Britons’ attitudes towards relationships. She said the liberal changes in mainstream values were reflected in the Government’s policies. The Tories, who introduced the much-derided Section 28 rule in 1988 that banned the promotion of same-sex relationships by local authorities, have since apologised and legislated in favour of gay marriage. But while we are more liberal about many things, playing away is not one of them.

“The findings are contradictory,” Ms Watt said. “Young Britons are still extremely conservative when it comes to having more than one partner at once, with almost all in this age group viewing it as always or mostly wrong.”

Britons still idealise having one faithful partner, according to the findings, even if that did not necessarily mean being married. A “rebel group” of just 6.4 per cent said they would consider having an open relationship. Ms Watt said: “It could be that people have a problem with non-monogamy because it can be seen to involve lying. I just think there is a real jealousy issue. We just don’t like the idea that whoever is sleeping with us is sleeping with someone else. We are one of the few species that is monogamous.”

Harry Benson, of the Marriage Foundation, a think-tank, said the hardening of support for stable exclusive relationships could be a backlash against soaring divorce rates in the 1960s and 1970s. “Everybody wants stable love,” he said. “It may be that we have a whole generation and more who have grown up experiencing the result of family breakdown. They have seen their parents split up and they don’t want it to happen to them.”