Adult men (n=156) and women (n=192) completed thrice-daily electronic diaries assessing individual- and partner-specific attributes and non-coital or coital sexual behaviours. Sexual motivations were: interest in sex, feeling in love with partner, wanted to have sex and partner wanted to have sex. The outcome variable was: sexual behaviour type (no sex, one vaginal sex event, one vaginal sex event+any other sex types, multiple vaginal sex events, any other sex types). Mixed-effect multinomial logistic regression modelled the influence of each sexual motivation on sexual behaviour type (Stata; all p<0.05). 'No sex' was the referent in all models; all models controlled for gender. Participants contributed 14856 total partner-associated diary entries. Most (54%; women: 56.5%, men: 51.2%) were associated with no sex; when sex occurred, the most common behaviour type was one vaginal sex event (13.1%) for women and other sex types (16.4%) for men. Wanting to have sex or perceiving a partner wanted to have sex were the strongest predictors of sexual behaviour type, and were associated with a greater number of reported sexual behaviours. Event-specific sexual motivations are associated with the choice to have sex, and with variation in the chosen sexual behaviours.