Background: Understanding the signification of the word ‘sex’ has implications for both medical research and clinical practice. Little is known about how people of varying ages define sex and how situational qualifiers influence definitions across age groups. To our knowledge, this is the first study of a representative sample to assess attitudes about which sexual behaviours constitute having ‘had sex’ and to examine possible mediating factors (gender, age, giving/receiving stimulation, male ejaculation, female orgasm, condom use or brevity). Methods: A telephone survey of English-speaking residents of Indiana (USA) using random-digit-dialling produced a final sample of 204 men and 282 women (n = 486) ranging in age from 18 to 96 years. Questions assessed the respondents’ attitudes on manual-genital (MG), oral-genital (OG), penile-vaginal intercourse (PVI) and penile-anal intercourse (PAI) behaviours. Results: There was no universal consensus on which behaviours constituted having ‘had sex’. More than 90% responded ‘yes’ to PVI but one in five responded ‘no’ to PAI, three in 10 responded ‘no’ to OG and about half endorsed MG. Fewer endorsed PVI with no male ejaculation (89.1%) compared with PVI without a qualifier (94.8%, P < 0.001). MG was endorsed more often when received (48.1%) than given (44.9%, P < 0.001). Among men, the oldest and youngest age groups were significantly less likely to believe certain behaviours constituted having ‘had sex’. Conclusions: These findings highlight the need to use behaviour-specific terminology in sexual history taking, sex research, sexual health promotion and sex education. Researchers, educators and medical practitioners should exercise caution and not assume that their own definitions of having ‘had sex’ are shared by their research participants or patients.