Purpose To investigate the extent to which need for intimacy in relationships and motives for sex are capable of distinguishing between adolescents who always have protected sex and adolescents who do not, with reference to both steady and casual relationships. Methods Seven-hundred-and-one adolescents (424 males and 277 females) aged 15 to 23 years filled out a questionnaire assessing determinants of sexual behavior and various outcome behaviors, such as having casual sex and condom use. Results Two-thirds of the participants (470) were sexually experienced. Discriminant functions using constructs of the Theory of Planned Behavior (attitude, subjective norm, self-efficacy), as well as the need for intimacy in relationships and motives for sex, were found to distinguish significantly between adolescents who always had protected sexual intercourse and adolescents who did not. This was valid both for sex within steady and within casual relationships. Besides a positive attitude and high perceived subjective norms, protected sex with a steady partner was characterized by low scores on the scale for the motive for sex to express love, and on the scale for the need for intimacy in relationships. Consistent condom use with casual partners was related to high self-efficacy, attitude, and perceived subjective norms, as well as a greater need for intimacy in relationships. Gender differences emerged with respects to determinants of (un)safe sex with casual partner(s), in that for males the variables of the Theory of Planned Behavior were most important in distinguishing those who had unsafe casual sex from those who had safe casual sex. For females, on the other hand, the variables concerning the meaning attached to having sex or to the relationship within which sex occurs seemed to be of more significance in explaining (un)safe sex with casual partner(s). However, owing to small subsamples, caution is warranted when interpreting these differences. Conclusion In adolescence, the factors which influence the decision to have (un)protected sex depend on the kind of relationship that exists between the partners. In addition, the goals which adolescents pursue with regard to their relationships and with regard to having sex fulfill an important role in the decision-making process whether to use condoms.