Today, the comparison of male homosexuality to bestiality is unfortunately too well-known from homophobic polemics. Yet this comparison has a history in the Anglophone world, and it emerged in the early European Middle Ages seemingly not in order to dehumanize men who had sex with men but in order to make bestiality appear serious by comparing it to male-male sexual acts. The eighth-century Paenitentiale Theodori —which collects the judgments of the Byzantine-born Archbishop Theodore—is the earliest extant English text to connect male-male sexual acts with bestiality. This comparison does not occur in the previous penitentials, but, after its appearance in the Paenitentiale , this comparison traveled throughout Western Europe. No scholarship to date examines the global origins of such a comparison. This paper argues that later medieval views of bestiality as perverse and as a serious sexual offense emerged from bestiality’s early comparison to same-sex acts (rather than vice-versa). Prior to the Paenitentiale Theodori , European theologians described bestiality as a minor sin akin to masturbation. Theodore borrowed the comparison of bestiality and male-male sex acts from a Latin mistranslation of the 314 Greek Council of Ancyra and from the Byzantine theologian St. Basil the Great. Since the early penitentials accorded male-male sexual acts some of the most serious penances, the comparison of bestiality to these acts elevated bestiality for the first time in Western Europe to the status of a serious and unnatural sin. Through connection to effeminizing male-male sexual acts, bestiality gained a reputation as a serious, boundary-violating sin in its own right.