Sexual mixing patterns among male-female partnerships in Melbourne, Australia

Background: Individuals who have both opposite- and same-sex partners have the potential to pass sexually transmitted infections (STIs) between high- and low-risk populations. Our aim was to examine assortative sexual mixing in terms of same-sex activity among male-female partnerships. Methods: This was a retrospective repeated cross-sectional study of male-female partnerships attending the Melbourne Sexual Health Centre (MSHC) from 2015 to 2019. Sex of sexual partners was collected via computer-assisted self-interview. We calculated the proportion of partnerships where at least one individual reported same-sex partners in the previous 12 months and the degree of assortativity by bisexuality. Results: A total of 2112 male-female partnerships (i.e. 4224 individuals) were included, with a median age of 27 years (IQR 23-31). Overall, 89.3% (1885/2112) of male-female partnerships did not report any other same-sex partners; however, in 9.5% (201/2112) of partnerships, same-sex partners were reported by one individual and in 1.2% (26/2112) of partnerships, both individuals reported same-sex partners. Bisexuality appeared to be slightly assortative in male-female partnerships (r = 0.163, 95% CI: 0.150-0.176; P < 0.001). Conclusion: One in 10 individuals in male-female partnerships had at least one same-sex partner within the previous 12 months. Individuals were minorly selective by bisexuality, suggesting the patterns of bisexual mixing in male-female partners are more variable and this may have a significant impact on STI transmission in heterosexual populations.