Background Attitudes towards sex and relationships influence sexual behaviour, the wellbeing of those maraginalised, and provision and use of services. Some countries conduct repeated surveys on sexual attitudes, but no longitudinal studies have investigated individuals’ changes in opinion over time. We have quantified consistency and change in acceptance of same-sex partnerships between ages 26 and 38 years, in New Zealand.

Methods Members of the Dunedin Study birth cohort answered computer-presented questions on their opinions about sexual and reproductive topics, including acceptability of same-sex partnerships, using items from the UK’s National Survey of Sexual Attitudes and Lifestyles (Natsal-1). Opinions from the age 26 and age 38 assessments were compared for consistency and change, and associated characteristics.

Results Response level was >90% at each assessment (n = 966; n = 936). The distribution of women’s opinions was similar for sex between men and sex between women, and liberalised with age. Men’s acceptance of sex between men was lower, and did not increase with age. At age 38 vs 26, 38% vs 38% of men and 58% vs 54% of women reported sex between two men was ‘rarely wrong/not wrong at all’. For sex between women, 58% vs 55% of men and 60% vs 57% of women reported this level of acceptance. Although aggregate changes were small, at an individual level 42% of men and 35% of women changed their opinion about sex between men, and about sex between women. For men and women, consistent acceptance of same-sex partnerships was associated having some same-sex experience, and more education.

Conclusions A fairly stable level of acceptance of same-sex sexual behaviour over 12 years was seen in this cohort but it obscures considerable flux in opinions among individuals. Better understanding of what influences opinions to become more or less liberal with age could inform health promotion.

Disclosure of interest This work was supported by the Health Research Council of New Zealand [12/1086]. The authors have no conflicts of interest.