Background Regular sexual partnerships among gay and bisexual men (GBM) who practice condomless anal intercourse (CLAI) have not been well characterised in terms of partnership type, HIV seroconcordance and risk of HIV transmission. Primarily sexual regular partnerships, although commonly reported by gay men, have largely been ignored in research and HIV prevention. Among regular partners reporting CLAI with each other, we determined factors differentiating romantic or committed relationships from partnerships organised primarily around sex (‘fuckbuddies’) and estimated the proportion of CLAI presenting risk for HIV transmission. Methods: An online, cross-sectional survey of Australian GBM was conducted. Univariate and multivariate generalised estimating equations were used to determine statistical associations. Results: Men reported on 2250 regular sexual partnerships. Over half the partnerships were romantic or committed relationships. Over half the partnerships were HIV-negative seroconcordant (54.9%), 3.1% were HIV-positive seroconcordant, 5.2% were serodiscordant and 36.8% were of unknown seroconcordance. Potential risks presented by CLAI were sometimes mitigated by protective factors, such as having a clear spoken agreement about sex with outside partners, having fewer outside partners, openly discussing HIV risk and having an agreement to reduce risk from outside partners. These protective factors were more often found in romantic or committed relationships than among primarily sexual partnerships, and were less often found in partnerships of unknown seroconcordance. Conclusion: CLAI is more common among regular sexual partnerships considered to be of a romantic, committed nature. However, factors associated with such romantic or committed partnerships can also protect against HIV transmission risk. Unknown seroconcordance, particularly lack of communication about HIV status among primarily sexual partnerships, is a key risk factor that needs to be addressed by HIV education.