What does it mean to have sex with someone? That was a question posed by researchers at Indiana University, who found that definitions differ widely.
They questioned almost 500 men and women aged between 18 and 96, and found that 95 per cent agreed penile-vaginal intercourse was sex.
However, around 30 per cent of respondents did not consider oral sex to be sex, while 20 per cent didn’t count anal sex in their definition.
Just over one in ten of those questioned said intercourse was not sex unless it ended in male ejaculation.
Dr William Yarber, co-author of the study, said the results show there is a "vagueness" in today’s culture and media about what sex actually is, which could have consequences in terms of sexual health.
"If people don’t consider certain behaviours sex, they might not think sexual health messages about risk pertain to them," he explained.
According to a recent survey by the MAC AIDS Fund, UK women may be putting themselves at risk because they don’t believe HIV is something that affects their community.