Background We investigated the age-specific distribution of genital HPV and HSV-2 antibodies among men whose sexual behaviour was confined either to men or to women (fixed) vs men whose sexual behaviour included both sexes (fluid). We hypothesised that fluid men may have more exposure to viral STIs.

Methods A total 4123 men from São Paulo, Brazil, Cuernavaca, Mexico, and Tampa, USA, aged 18–70 years, reported their sexual behaviour every six months for 2 years. Analysis was restricted to 1412 Brazilian men where fluidity had a much higher prevalence. Genital exfoliated cells for HPV PCR genotyping were collected every 6 months and blood for HSV-2 antibody testing was collected every 12 months. We used chi square and the Cochran-Armitage test to assess associations and age-specific linear trends.

Results Of 1292 men at enrollment who provided sexual behaviour information at ≥ 2 time points, those aged 17–24 reported the most fluidity (24%) and those aged 50–70 reported the least (16%) although the linear trend was not significant (p = 0.12). After two years of follow up, HPV prevalence was comparable and stable by age among both fixed and fluid men for any HPV type, oncogenic types, and nononcogenic types. Prevalence of HSV-2 antibodies was higher among fluid than fixed men (65% and 59%, respectively) although not significantly different (p = 0.06). The prevalence of HSV-2 antibodies increased by age among both fixed and fluid men (p for trend < 0.0001 and = 0.0006, respectively).

Conclusions These data suggest that fluidity is not associated with increased prevalence of HPV and HSV-2. There was no trend for genital HPV by age which might be expected of a highly prevalent viral infection that often escapes immune surveillance; however, age was associated with HSV-2 antibodies as would be expected of a highly prevalent viral infection that less often escapes immune recognition.