ABSTRACTData from 1,380 self-identified bisexual women and men age 18 to 70 living in the United States document significant diversity across age and gender in attitudes regarding which behaviors constitute “sex” and behavioral histories. With the exceptions of deep kissing, penile-vaginal intercourse, and penile-anal intercourse, the 18 to 29 age groups of both genders were significantly less likely to count all other behaviors (manual and oral stimulation of breasts, genitals, and anus; and use of sex toys) as sex. Stronger age effects were found for men than women for manual and oral stimulation of genitals or anus, and sex toy use. Compared to older men and women, fewer 18- to 29-year-olds had experienced each behavior. Use of behaviorally nonspecific sexual history questions in research and clinical practice is problematic because (1) across age-group comparisons may be contaminated by differential interpretation of questions across age groups and (2) within age-group comparisons may be confounded by gender differences in interpretation.