There has been a long-running debate as to whether sexual cultures in sub-Saharan Africa are permissive or characterised by restrictive rules, rituals and self-restraint. This paper, based on participant observation data, outlines the main features of sexual culture in rural northern Tanzania and highlights both permissive and restrictive norms and expectations for young people. It also illustrates how sexual beliefs are socially constructed and subject to social change. Sexual activity is constrained by clear norms of school pupil abstinence, female sexual respectability and taboos around the discussion of sex. However, these norms are incompatible with several widely held expectations: that sexual activity is inevitable unless prevented, sex is a female resource to be exploited, restrictions on sexual activity are relaxed at festivals, and masculine esteem is boosted through sexual experience. Differential commitment to these norms and expectations reflects conflicts between generations and genders. Young people appear to manage the contradictions in these norms by concealing their sexual relationships. This almost certainly contributes to their short duration and the high levels of partner change, since relationships are not reinforced through social recognition and there is little scope to develop intimacy through non-sexual contacts.