Background Transmission of HIV and other Sexually Transmitted Infections (STI) is ongoing in Western populations of Men who have Sex with Men (MSM). The main indicator of sexual risk is unprotected anal intercourse (UAI) with casual sex partners. It was suggested that group sex might be a high risk setting for HIV and STI transmission. Aim of the present study is to identify differences in sexual risk behaviour and STI-rates among MSM engaging in group sex compared to one-on-one casual sex.

Methods We used cross-sectional data derived through 6-monthly questionnaires and STI screening (infectious syphilis, gonorrhoea or Chlamydia) from the Amsterdam Cohort Studies (ACS) between December 2008 and December 2009. The study population (n=310) consisted of HIV-negative MSM who reported having engaged in both group sex and one-on-one sex (n=119) and MSM reporting one-on-one sex only (n=191). To identify differences in sexual risk behaviour and STI-rates between MSM engaging in group sex and one-on-one sex, we used χ² tests, Mann−Whitney U tests and logistic regression analyses, corrected for repeated measures.

Results MSM engaging in both group sex and one-on-one sex had less anal intercourse (AI) during group sex (79/119; 66.4%) compared to during one-on-one sex (106/119; 89.1%). Moreover, they were less likely to have UAI during group sex compared to one-on-one sex (OR adj 0.44; 95% CI 0.26 to 0.74). Men engaging in one-on-one sex only reported less AI (129/191; 67.5 %), while UAI-levels were similar (OR adj1.47; 95% CI 0.82 to 2.62) compared to one-on-one sex of men who also engage in group sex. Drug use during sex was associated with UAI (OR adj1.78; 95% CI 1.03 to 3.09), independent of whether sex took place in a group sex or one-on-one sex setting. Finally, men engaging in group sex were more likely to be diagnosed with any STI (13.4% vs 5.1%; p=0.013) compared to men engaging in one-on-one sex only. For men engaging in one-on-one sex only, but not for men engaging in group sex, UAI was associated with being diagnosed with any STI (OR 8.44; 95 % CI 2.0 to 35.2).

Conclusions The group sex setting might not pose a threat for HIV-infection in MSM, as UAI-rates were lower during group sex compared to one-on-one sex. However, although they did not report more UAI, men engaging in group sex had higher STI-rates than men engaging in one-on-one sex only. This indicates that men engaging in group sex practice other sexual techniques than AI that contribute to acquiring STI.