Based on interviews with 34 men, almost all of whom have unprotected sex with men most or all of the time, this paper documents the interactional process, narrative elements and meaning construction in situations of 'bareback' sex. Narratives show the differentiated cultural capital circulating among distinct circuits of gay and bisexual men that define the taken-for-granted rules of conduct for sexual interactions and give rise to high-risk situations. Many of the positive men speak of being part of a social environment where 'everybody knows' a set of rules whereby sex without condoms can happen as a default circumstance to be interrupted only when a partner asserts a need to protect himself. The practical reasoning processes and interactional back-and-forth in the unfolding of sexual interactions, both on the internet and in person, show the uneven and fallible accomplishment of sero-sorting and the generation of situations of high HIV risk and vulnerability when men from different micro-cultures encounter each other.