Across human societies, a fraction of men and women (around 2-10%) report engaging in sex with samesex partners, either exclusively or, more frequently, in addition to having heterosexual partners (bisexuals) (ACSF. Nature 1992; Melbye et al. Am J Epidemiol 1992; Bailey et al. Psychol Sci Public Interest 2016; Semenyna et al. J Sex Res 2017) (Fig. 1).