With sex infections rising sharply in the UK, couples have been warned to have fewer lovers and to stop having "overlapping" relationships.
Health experts have issued the warnings in a bid to halt the spread of sex infections.
According to last year's reports, cases of chlamydia rose six per cent.
The Health Protection Agency (HPA) called for under 25s to be screened for it when they change lovers.
It also urged everyone to use a condom with a new partner until they have both been screened.
And it wants all gay men to take an annual HIV test.
"It's increasingly the case that among young people casual sex is part of life," The Sun quoted Professor Peter Borriello, of the HPA, as saying.
"A shift in behaviour is the only way to bring down these infections," he added.